Exclude chrome.js from the default externs - move to an environment

[ChadKillingsworth]

The chrome.js externs are becoming problematic because:

a) they are not standard browser externs
b) they cause naming collisions (see #1180, #1189)

The chrome externs should be moved to a custom environment. However, since a chrome extern set also needs the browser externs, a little bit of work would be required to do this.

[jxpx777]

Any update on this? If you're using closure compiler for building Firefox extensions, these built-in externs conflict with any other definition of the Tab type. Our externs for Firefox's add-on SDK include the following definitions for the tabs module:

// tabs module
var tabs = {};
/** @constructor */ function Tab(){};
Tab.prototype.pin = function(){};
Tab.prototype.unpin = function(){};
/** @param {function()} callback */
Tab.prototype.close = function(callback) {};
Tab.prototype.reload = function(){};
Tab.prototype.activate = function(){}
Tab.prototype.getThumbnail = function(){};
/** @param {{contentScriptFile: (string|Array<string>), contentScript: (string|Array<string), contentScriptOptions: !Object=, onMessage: Function=, onError: Function=}}*/
Tab.prototype.attach = function(options) {};
/** @type {string} */Tab.prototype.id;
/** @type {string} */Tab.prototype.url;
/** @type {string} */Tab.prototype.title;
/** @type {string} */Tab.prototype.favicon;
/** @type {string} */Tab.prototype.contentType;
/** @type {string} */Tab.prototype.readyState;
/** @type {number} */Tab.prototype.index;
/** @type {boolean} */Tab.prototype.isPinned;
/** @type {Window} */Tab.prototype.window;

/** @type {Tab} */tabs.activeTab

Building with this externs file included causes the following errors because closure compiler wrongly assumes that the Tab type for Chrome is the only possible definition of a Tab type.

GCC encountered problems minifying /Users/jp/tmp/opx/1Password.safariextension/data/src/end.min.js:
GoogleClosureCompilerExterns/jetpack.externs.js:125: WARNING - variable Tab redefined, original definition at externs.zip//chrome.js:341
/** @constructor */ function Tab(){};
                             ^^^

GoogleClosureCompilerExterns/jetpack.externs.js:133: WARNING - Bad type annotation. expected closing } See https://github.com/google/closure-compiler/wiki/Bad-Type-Annotation for more information.
/** @param {{contentScriptFile: (string|Array<string>), contentScript: (string|Array<string>), contentScriptOptions: !Object=, onMessage: Function=, onError: Function=}}*/
                                                                                                                            ^

GoogleClosureCompilerExterns/jetpack.externs.js:135: WARNING - variable Tab.prototype.id redefined with type string, original definition at externs.zip//chrome.js:346 with type number
/** @type {string} */Tab.prototype.id;
                     ^^^^^^^^^^^^^^^^

I realize there is the --env=CUSTOM option, but this results in dozens of warnings in our project and failure to define things like DOM element interfaces (Element, HTMLInputElement) and Window, which is clearly not a workable situation.