Add a mutex to GrContext::readSurfacePixels to protect against multiple CPU raster threads accessing the same GrContext to read back GPU input data

BUG=chromium:524717

TBR=reed@google.com

Committed: https://skia.googlesource.com/skia/+/eb662bc407cec0585a821946fef123102cae64db

Review URL: https://codereview.chromium.org/1329313002
2 files changed