Roll external/skia b3f6654a5..9b86955c6 (4 commits)

https://skia.googlesource.com/skia.git/+log/b3f6654a5..9b86955c6

2019-05-13 herb@google.com Add thread safety annotations for SkMutex
2019-05-13 robertphillips@google.com Alter createTestingOnlyBackendTexture methods
2019-05-13 csmartdalton@google.com ccpr: Clip path octo bounds by the scissor
2019-05-13 bsalomon@google.com Add color space and type conversion support to asyncReadPixels

The AutoRoll server is located here: https://autoroll-internal.skia.org/r/android-master-autoroll

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.

Test: Presubmit checks will test this change.
Change-Id: I8aab96337f71b5e93d3204b0d4528f65a93ecabf
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.