[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
Review URL: https://codereview.chromium.org/1512183002

Cr-Commit-Position: refs/heads/master@{#364410}
(cherry picked from commit dea09d3b7ac5fd50ca7ba3baa70fb37920895487)

Committed: https://chromium.googlesource.com/chromium/src/+/c0118aff422c78dd244e7ef6d877ce5ef7874441

[maxbogue, 2016-01-12 20:24:58]

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

Review URL: https://codereview.chromium.org/1512183002

Cr-Commit-Position: refs/heads/master@{#364410}
(cherry picked from commit dea09d3b7ac5fd50ca7ba3baa70fb37920895487)
==========

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

Review URL: https://codereview.chromium.org/1512183002

Cr-Commit-Position: refs/heads/master@{#364410}
(cherry picked from commit dea09d3b7ac5fd50ca7ba3baa70fb37920895487)

Committed:
https://chromium.googlesource.com/chromium/src/+/c0118aff422c78dd244e7ef6d877...
==========

[maxbogue, 2016-01-12 20:24:59]

Committed patchset #1 (id:1) manually as
c0118aff422c78dd244e7ef6d877ce5ef7874441.