content: Always run EnforceMemoryPolicy() callbacks on the same thread.

content: Always run EnforceMemoryPolicy() callbacks on the same thread.

This allows ::ScheduleEnforceMemoryPolicy() to be called on
a worker thread without a message loop.

BUG=549274
TEST=content_unittests --gtest_filter=HostDiscardableSharedMemoryManagerScheduleEnforceMemoryPolicyTest.SetMemoryLimitOnSimpleThread

Committed: https://crrev.com/d13710832e9d0e6302cf8388cda94a7a780d8664
Cr-Commit-Position: refs/heads/master@{#356989}

[reveman, 2015-10-29 21:32:17]

The CQ bit was checked by reveman@chromium.org to run a CQ dry run

[reveman, 2015-10-29 21:32:45]

reveman@chromium.org changed reviewers:
+ avi@chromium.org

[commit-bot: I haz the power, 2015-10-29 21:34:51]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1430743002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1430743002/1

[Avi (use Gerrit), 2015-10-29 22:57:44]

lgtm

[commit-bot: I haz the power, 2015-10-29 23:36:15]

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

[commit-bot: I haz the power, 2015-10-29 23:36:15]

Dry run: This issue passed the CQ dry run.

[reveman, 2015-10-29 23:48:21]

The CQ bit was checked by reveman@chromium.org

[commit-bot: I haz the power, 2015-10-29 23:48:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1430743002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1430743002/1

[commit-bot: I haz the power, 2015-10-29 23:56:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-29 23:57:18]

Patchset 1 (id:??) landed as
https://crrev.com/d13710832e9d0e6302cf8388cda94a7a780d8664
Cr-Commit-Position: refs/heads/master@{#356989}