Roll external/skia 7f9b21569..51245935f (6 commits)
https://skia.googlesource.com/skia.git/+log/7f9b21569..51245935f
2017-10-12 jvanverth@google.com Fix assert in GrSmallPathRenderer tickled by GLProgramsTest
2017-10-12 fmalita@chromium.org Plumb more SkPaintFilterCanvas virtuals
2017-10-12 mtklein@chromium.org abort SkRRect::transform() before dst->scaleRadii() asserts
2017-10-12 hcm@google.com Update Skia milestone to 64
2017-10-12 stani@google.com Implement getGrContext in SkPaintFilterCanvas
2017-10-12 jvanverth@google.com Batch better in GrSmallPathRenderer and add perspective support.
Test: Presubmit checks will test this change.
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.
Test: Ran Android HWUI on a non overlapping View with alpha,
Bug: 66988832
Change-Id: Id5fb440fdc76b2684a966225baf135047b7670ae