[Sync] Make InvalidationPreferences.commit() thread-safe.

[Sync] Make InvalidationPreferences.commit() thread-safe.

The lock is static because we make many instances of
InvalidationPreferences but only one can be committing at a time.

BUG=568369
Committed: https://crrev.com/dea09d3b7ac5fd50ca7ba3baa70fb37920895487
Cr-Commit-Position: refs/heads/master@{#364410}

[maxbogue, 2015-12-10 02:12:10]

maxbogue@chromium.org changed reviewers:
+ nyquist@chromium.org

[maxbogue, 2015-12-10 02:12:10]

Tommy, PTAL.

[nyquist, 2015-12-10 18:27:43]

lgtm

[maxbogue, 2015-12-10 18:33:06]

The CQ bit was checked by maxbogue@chromium.org

[commit-bot: I haz the power, 2015-12-10 18:35:44]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1512183002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1512183002/1

[commit-bot: I haz the power, 2015-12-10 18:43:06]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-12-10 18:43:47]

Description was changed from

==========
[Sync] Make InvalidationPreferences.commit() thread-safe.

The lock is static because we make many instances of
InvalidationPreferences but only one can be committing at a time.

BUG=568369
==========

to

==========
[Sync] Make InvalidationPreferences.commit() thread-safe.

The lock is static because we make many instances of
InvalidationPreferences but only one can be committing at a time.

BUG=568369

Committed: https://crrev.com/dea09d3b7ac5fd50ca7ba3baa70fb37920895487
Cr-Commit-Position: refs/heads/master@{#364410}
==========

[commit-bot: I haz the power, 2015-12-10 18:43:48]

Patchset 1 (id:??) landed as
https://crrev.com/dea09d3b7ac5fd50ca7ba3baa70fb37920895487
Cr-Commit-Position: refs/heads/master@{#364410}