Remove mutex guards from GrContext::readSurfacePixels

Change-Id: Ief4f0faae7abeb996bdfc993e368e64755da15a7
Reviewed-on: https://skia-review.googlesource.com/5929
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed