Introduce MetricsServiceClient::StartGatheringMetrics

Introduce MetricsServiceClient::StartGatheringMetrics

This API is called when initial metrics gathering should start, and takes in a
callback that is called when initial metrics gathering is complete. It is
currently empty but will be filled in as initial metrics gathering code moves
out of MetricsService.

BUG=374231
R=asvitkine@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=272479

[Alexei Svitkine (slow), 2014-05-21 12:32:25]

lgtm

[blundell, 2014-05-22 15:10:02]

The CQ bit was checked by blundell@chromium.org

[commit-bot: I haz the power, 2014-05-22 15:10:50]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/blundell@chromium.org/299663011/1

[commit-bot: I haz the power, 2014-05-22 15:35:23]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  android_dbg_triggered_tests on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_dbg_triggered...)
  chromium_presubmit on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/chromium_presubmit/bu...)
  ios_rel_device_ninja on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/ios_rel_device_ninja/...)
  linux_chromium_chromeos_clang_dbg on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_chrome...)

[commit-bot: I haz the power, 2014-05-22 15:42:29]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-05-22 15:42:29]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/chromium_presubmit/bu...)
  ios_rel_device_ninja on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/ios_rel_device_ninja/...)

[blundell, 2014-05-23 08:18:45]

The CQ bit was checked by blundell@chromium.org

[commit-bot: I haz the power, 2014-05-23 08:19:32]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/blundell@chromium.org/299663011/20001

[blundell, 2014-05-23 10:56:00]

Committed patchset #2 manually as r272479 (presubmit successful).