This reverts commit c32f6614.

Reason for revert:
https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20TSAN/builds/20563

Original change's description:
> [logging] Remove unused code
> 
> This removes unused mutex, headers from log.(h|cc).
> 
> Change-Id: Ie4dc69a7efa9494b21ff6e2b19828bd740e3a47c
> Reviewed-on: https://chromium-review.googlesource.com/1041967
> Reviewed-by: Camillo Bruni <cbruni@chromium.org>
> Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52946}

TBR=jarin@chromium.org,cbruni@chromium.org

Change-Id: I48b63c7fc6a10e118b735f708db37d353ba6c0f7
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/1041969Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52950}