Remove SkThread.h, include SkMutex.h or SkAtomics.h as appropriate.

SkThread.h doesn't do anything anymore execept include those two,
and thankfully, it doesn't seem to be mentioned outside Skia.

No public API changes.
TBR=reed@google.com

BUG=skia:

Review URL: https://codereview.chromium.org/1215393002
56 files changed