[task scheduler] Remove scheduling mutex.
[task scheduler] Remove scheduling mutex.

BUG=skia:

Change-Id: I420a8b837921d19d48663f50832da9702100d930
Reviewed-on: https://skia-review.googlesource.com/6084
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Commit-Queue: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot