PSA: extensions_browsertests has been removed from the bots/builders

[James Cook]

TL;DR: I've just removed the deprecated test binary extensions_browsertests from the CQ/CI builders and testers. If you see failures please ping me or revert this CL.

I've also updated our documentation, so if you see a reference to extensions_browsertests please let me know so I can fix it.

Historical Background:

extensions_browsertests used to contain a small number of integration tests for extensions JS APIs. Those few tests have been moved to browser_tests (where most extensions API tests live) which provides a better simulation of the production extensions runtime. 

extensions_browsertests was also based on the long deprecated binary app_shell, an experimental runtime for Chrome Apps which never shipped. The extensions team would like to stop maintaining app_shelll, but we can't until test binaries that depend on it are removed.

Fun trivia: I wrote the initial version of app_shelll, ~10 years ago. I have come full circle and will shortly delete my work. :-)

Onward and upward,

James