Add UMAs about Blink GC metrics

Add UMAs about Blink GC metrics

This CL adds the following metrics to UMAs:

- Duration of time consumed in Heap::collectGarbage().
- Duration of time consumed in ThreadState::performPendingSweep().
- The total size of object space in all threads when Heap::collectGarbage() is triggered.

Corresponding CL is here: https://codereview.chromium.org/304673003/

BUG=340522
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=273539

[haraken, 2014-05-28 09:03:07]

Blink side CL is here:
https://codereview.chromium.org/304673003/

PTAL.

[kouhei (in TOK), 2014-05-28 09:04:13]

lgtm

+uma team

non-owner lgtm if presubmit pass

[Alexei Svitkine (slow), 2014-05-28 19:30:21]

LGTM

[haraken, 2014-05-28 23:15:02]

The CQ bit was checked by haraken@chromium.org

[commit-bot: I haz the power, 2014-05-28 23:15:30]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/haraken@chromium.org/302433014/40001

[commit-bot: I haz the power, 2014-05-29 03:32:11]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  android_dbg_triggered_tests on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_dbg_triggered...)
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

[kouhei (in TOK), 2014-05-29 03:32:38]

On 2014/05/29 03:32:11, I haz the power (commit-bot) wrote:
> FYI, CQ is re-trying this CL (attempt #1).
> The failing builders are:
>   android_dbg_triggered_tests on tryserver.chromium
>
(http://build.chromium.org/p/tryserver.chromium/builders/android_dbg_triggered...)
>   win_chromium_rel on tryserver.chromium
>
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

I think you can use NOTRY=true :)

[commit-bot: I haz the power, 2014-05-29 19:18:10]

Change committed as 273539