[autoroll] Fix mode change mutex
[autoroll] Fix mode change mutex

modeMtx is not necessary since the ModeHistory object has its own mutex.

Bug: skia:
Change-Id: I1ff83a92a10054d26b7c5723575cc7fbc7460688
Reviewed-on: https://skia-review.googlesource.com/23583
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@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