Extend browser.createUserContext with unhandledPromptBehavior parameter (#896)

Addressing #789

Analogue of `acceptInsecureCerts`, add `unhandledPromptBehavior` parameter to `browser.createUserContext`.

Author: sadym-chromium

index.bs

@@ -2713,7 +2713,8 @@ The <dfn export for=commands>browser.createUserContext</dfn> command creates a
 
       browser.CreateUserContextParameters = {
         ? acceptInsecureCerts: bool,
-        ? proxy: session.ProxyConfiguration
+        ? proxy: session.ProxyConfiguration,
+        ? unhandledPromptBehavior: session.UserPromptHandler
       }
       </pre>
    </dd>
@@ -2747,6 +2748,10 @@ The [=remote end steps=] with |session| and |command parameters| are:
       [=user context to accept insecure certificates override map=][|user context|]
       to |acceptInsecureCerts|.
 
+1. If |command parameters| [=map/contains=] "<code>unhandledPromptBehavior</code>",
+   [=map/set=] [=unhandled prompt behavior overrides map=][|user context|] to
+   |command parameters|["<code>unhandledPromptBehavior</code>"].
+
 1. If |command parameters| [=map/contains=] "<code>proxy</code>":
 
    1. Let |proxy configuration| be |command parameters|["<code>proxy</code>"].
@@ -3115,11 +3120,22 @@ A <dfn>viewport configuration</dfn> is a [=struct=] with an [=struct/item=] name
 <dfn attribute for="viewport-configuration">viewport</dfn> which is a [=viewport-configuration/viewport dimensions=]
 or null and an [=struct/item=] named <dfn attribute for="viewport-configuration">devicePixelRatio</dfn> which is a float or null.
 
+An <dfn>unhandled prompt behavior struct</dfn> is a [=struct=] with:
+* [=struct/Item=] named <dfn attribute for="unhandled-prompt-behavior-alert">alert</dfn> which is a string or null;
+* [=struct/Item=] named <dfn attribute for="unhandled-prompt-behavior-beforeUnload">beforeUnload</dfn> which is a string or null;
+* [=struct/Item=] named <dfn attribute for="unhandled-prompt-behavior-confirm">confirm</dfn> which is a string or null;
+* [=struct/Item=] named <dfn attribute for="unhandled-prompt-behavior-default">default</dfn> which is a string or null;
+* [=struct/Item=] named <dfn attribute for="unhandled-prompt-behavior-file">file</dfn> which is a string or null;
+* [=struct/Item=] named <dfn attribute for="unhandled-prompt-behavior-prompt">prompt</dfn> which is a string or null.
+
 A [=remote end=] has a <dfn>viewport overrides map</dfn> which is a weak map between [=user contexts=] and [=viewport configuration=].
 
 A [=remote end=] has a <dfn>locale overrides map</dfn> which is a weak map between
 [=navigables=] or [=user contexts=] and string or null.
 
+A [=remote end=] has an <dfn>unhandled prompt behavior overrides map</dfn> which is a
+weak map between [=user contexts=] and [=unhandled prompt behavior struct=].
+
 ### Types ### {#module-browsingcontext-types}
 
 #### The browsingContext.BrowsingContext Type #### {#type-browsingContext-Browsingcontext}
@@ -5721,6 +5737,27 @@ closed</dfn> steps given |window|, |type|, |accepted| and optional |user text|
    </dd>
 </dl>
 
+<div algorithm>
+To <dfn>get navigable's user prompt handler</dfn> given |type| and |navigable|:
+
+1. Let |user context| be |navigable|'s [=associated user context=].
+
+1. If [=unhandled prompt behavior overrides map=] [=map/contains=] |user context|:
+
+  1. Let |unhandled prompt behavior override| be [=unhandled prompt behavior overrides map=][|user context|].
+
+  1. If |unhandled prompt behavior override|[|type|] is not null, return
+     |unhandled prompt behavior override|[|type|].
+
+  1. If |unhandled prompt behavior override|[<code>"default"</code>] is not null,
+     return |unhandled prompt behavior override|[<code>"default"</code>].
+
+1. Let |handler configuration| be [=get the prompt handler=] with |type|.
+
+1. Return |handler configuration|'s [=prompt handler configuration/handler=].
+
+</div>
+
 <div algorithm>
 The [=remote end event trigger=] is the <dfn export>WebDriver BiDi user prompt
 opened</dfn> steps given |window|, |type|, |message|, and optional |default value|
@@ -5730,10 +5767,8 @@ opened</dfn> steps given |window|, |type|, |message|, and optional |default valu
 
 1. Let |navigable id| be the [=navigable id=] for |navigable|.
 
-1. Let |handler configuration| be [=get the prompt handler=] with |type|.
-
-1. Let |handler| be |handler configuration|'s [=prompt handler
-   configuration/handler=].
+1. Let |handler| be [=get navigable's user prompt handler=] with |type| and
+   |navigable|.
 
 1. Let |params| be a [=/map=] matching the
    <code>browsingContext.UserPromptOpenedParameters</code> production with the