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Since its introduction in Canary I became a big fan of the CSS rule usage
tracker and I'm looking forward to its introduction in Chrome stable.

I also thought it would be a great idea to run it periodically as part of the
e2e tests at work to collect the (un)used CSS rules instead of during manual
testing.

I then decided to implement a POC [1] to run the CSS tracker with Protractor.
This of course required some changes in Chromedriver, Protractor and WebDriver
JS Extender.
In particular, I had to change the Chromedriver so that I could send the
commands to start and stop the CSS rule usage tracking
(CSS.startRuleUsageTracking/CSS.stopRuleUsageTracking) to the remote debugger.

I thought of 3 possible implementations:
1) as part of the performance logging [2];
2) as a new endpoint of the Chromedriver to start/stop the CSS rule usage
tracking;
3) as a new endpoint of the Chromedriver to send any command directly to the
debugger, hence opening up the road to a whole series of tools able to send
commands directly to the debugger through the Chromedriver.

Of these 3 options, I implemented option 2 and 3 in this change.

If this change is acceptable, I'll proceed with the tests and fixing the linter issues.

Thanks,

EM.

[1] https://github.com/ventuno/css-usage-recorder/tree/ftr-css-recording
[2]
https://sites.google.com/a/chromium.org/chromedriver/logging/performance-log

BUG=

Review-Url: https://codereview.chromium.org/2743013002
Cr-Commit-Position: refs/heads/master@{#469275}