Make media::WallClockTimeSource thread safe.

Make media::WallClockTimeSource thread safe.

Temporary measure until both Pipeline is reworked to provide a time
estimate to the main thread and VideoRendererImpl's internal thread
is eliminated.

This is similar to the the locking currently present in
AudioRendererImpl.

BUG=412764
Committed: https://crrev.com/465cf3e643d44ce7176072bdbac1a9b03ac70514
Cr-Commit-Position: refs/heads/master@{#294207}

[scherkus (not reviewing), 2014-09-10 17:39:21]

scherkus@chromium.org changed reviewers:
+ xhwang@chromium.org

[xhwang, 2014-09-10 18:07:25]

lgtm

[scherkus (not reviewing), 2014-09-10 18:34:53]

The CQ bit was checked by scherkus@chromium.org

[commit-bot: I haz the power, 2014-09-10 18:38:24]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/scherkus@chromium.org/562673003/1

[commit-bot: I haz the power, 2014-09-10 19:18:50]

Committed patchset #1 (id:1) as 78aaabeef255995f7d7414c2f66df899cd940fc0

[commit-bot: I haz the power, 2014-09-10 19:27:41]

Patchset 1 (id:??) landed as
https://crrev.com/465cf3e643d44ce7176072bdbac1a9b03ac70514
Cr-Commit-Position: refs/heads/master@{#294207}