Added UMA instrumentation for site-instrumented user timing marks and measures

Added UMA instrumentation for site-instrumented user timing marks and measures

Added UMA histograms for each performance.mark (PLT.UserTiming_Mark) and merformance.measure (PLT.UserTiming_MeasureDuration) that were instrumented by pages.

Entry points were also exposed that measure the last mark on a page as well as if a page was instrumented for user timing so that the Chrome side of the code can report UMA's to track those.  The last mark (PLT.UserTiming_LastMark) is useful in cases where sites instrument a bunch of things they want to track and it lets you know when the last one of those happened (like all of the above-the-fold images finished loading).  Knowing if a page was instrumented (PLT.UserTiming_Instrumented) with user timing will let us track adoption over time.

The Chrome-side of the code will be uploaded after the blink patch lands and is included in the blink milestone used by Chrome.

BUG=262567
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=156481

[Pat Meenan, 2013-08-16 16:37:14]

It turns out that someone had already plumbed UMA histograms into blink so
adding the instrumentation to the user timing marks/measures was
straightforward.

The plan here is to get sites that have instrumented the things that they care
about with their own RUM solutions to also use the standard user timing
interfaces and for us to be able to track the performance of those
explicitly-instrumented metrics.

A lot of big web properties have their own custom instrumentation and I'm pretty
sure I can convince them of the benefits for using the standard interface if we
are watching those metrics as we work on Chrome perf (a perfect example is that
it would have caught a regression we introduced with the resource prioritization
that slowed down a pretty important site that had done a lot of hand
optimizations).

I'm also exposing the user timing marks in WebPagetest and am getting commercial
RUM providers to leverage it so this should help close the loop:
http://blog.patrickmeenan.com/2013/07/measuring-performance-of-user-experienc...

[James Simonsen, 2013-08-16 22:17:02]

We already have a use counter in WebCore.FeatureObserver. It's
UnprefixedUserTiming. You can see it here:

https://uma.googleplex.com/histograms?dayCount=1&endDate=08-16-2013&version=E...

[Pat Meenan, 2013-08-17 11:58:08]

I removed the usage reporting (since it is already handled) and also removed the
tracking of the last mark.

We can wait to see how people implement user timing on their sites before
deciding if and what roll-ups make sense and removing the last mark tracking
reduces it to a completely trivial patch.

[tonyg, 2013-08-17 14:14:41]

lgtm

Thanks :)

[commit-bot: I haz the power, 2013-08-21 15:50:47]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/pmeenan@chromium.org/23245004/5001

[commit-bot: I haz the power, 2013-08-21 17:31:55]

Change committed as 156481