Add --fast-build flag to automate.py tool to speed up build times

[cztomczak]

That flag should do the following:

• disable compile and link optimizations by setting is_official_build=false
• maybe disable nacl to speed up build further by setting enable_nacl=false
• maybe set other flags like is_debug=false (no, see below), symbol_level=0 and remove_webcore_debug_symbols=true

This chromium page explains common GN args:
https://www.chromium.org/developers/gn-build-configuration

CEF default args:
https://github.com/chromiumembedded/cef/blob/master/tools/gn_args.py

In Chromium:

• Turning off symbols with symbol_level=0 will make builds faster, however debugging is not possible then
• Turning off native client with enable_nacl = false will make build faster. CEF sets this to false by default.
• Removing webcore symbols with remove_webcore_debug_symbols=true will make build faster on Linux and Mac. Do not set it to true on Windows, as it will make builds slower.
• By default is_component_build=true is set which makes debug builds faster. It requires is_debug=True and in CEF requires is_official_build=false to have these faster builds thanks to component build. However CEF sets is_component_build=false by default, because it is required to create CEF binary distribution.

In CEF:

• CEF by default sets is_debug=true unless it detects is_official_build=true and in such case it sets is_debug=false.
• With is_official_build=true CEF will set dcheck_always_on=true and thus CEF's "official Debug" build is a Release build with dchecks and symbols.

A recent commit aec9f62 increased the default ninja jobs and it should speed up builds significantly as well.