Add a field trial to disable deferred committing of cookie changes

Add a field trial to disable deferred committing of cookie changes
by prerender. This can be used to emergency disable the feature, or to do
comparisons of other metrics.

R=davidben@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=272824

[davidben, 2014-05-23 18:02:09]

lgtm

[tburkard, 2014-05-23 20:08:48]

The CQ bit was checked by tburkard@chromium.org

[commit-bot: I haz the power, 2014-05-23 20:09:23]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tburkard@chromium.org/293123010/1

[commit-bot: I haz the power, 2014-05-24 00:32:54]

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:
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

[commit-bot: I haz the power, 2014-05-24 00:42:43]

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

[commit-bot: I haz the power, 2014-05-24 00:42:43]

Try jobs failed on following builders:
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

[tburkard, 2014-05-24 05:52:20]

The CQ bit was checked by tburkard@chromium.org

[commit-bot: I haz the power, 2014-05-24 05:52:45]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tburkard@chromium.org/293123010/20001

[commit-bot: I haz the power, 2014-05-24 08:09:07]

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_aosp on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_aosp/builds/7...)

[commit-bot: I haz the power, 2014-05-24 08:15:24]

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

[commit-bot: I haz the power, 2014-05-24 08:15:25]

Try jobs failed on following builders:
  android_aosp on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_aosp/builds/7...)

[tburkard, 2014-05-26 09:39:45]

The CQ bit was checked by tburkard@chromium.org

[commit-bot: I haz the power, 2014-05-26 09:40:53]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tburkard@chromium.org/293123010/30001

[commit-bot: I haz the power, 2014-05-26 11:06:03]

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:
  win_chromium_compile_dbg on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_compile_...)
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)
  win_chromium_x64_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_x64_rel/...)

[tburkard, 2014-05-26 15:46:32]

Committed patchset #3 manually as r272824 (presubmit successful).