This removes unused mutex, headers from log.(h|cc).

Change-Id: Ie4dc69a7efa9494b21ff6e2b19828bd740e3a47c
Reviewed-on: https://chromium-review.googlesource.com/1041967Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52946}