Roll external/skia 9a8b846baf43..72d2f571ff2a (5 commits)

https://skia.googlesource.com/skia.git/+log/9a8b846baf43..72d2f571ff2a

2019-09-19 mtklein@google.com stifle warning in tValue2Scalar()
2019-09-19 jvanverth@google.com Set up prototype interface for MakeFromCAMetalLayer
2019-09-19 robertphillips@google.com Revert "Remove a usage of GrPixelConfig in SkGpuBlurUtils::GaussianBlur"
2019-09-19 csmartdalton@google.com Use GrResourceProvider::MakeApprox for mask filter proxy sizes
2019-09-19 robertphillips@google.com Remove a usage of GrPixelConfig in SkGpuBlurUtils::GaussianBlur

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://skia-autoroll.corp.goog/r/android-master-autoroll
Please CC nifong@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

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

Test: Presubmit checks will test this change.
Change-Id: I3aa06e0391a114f5c2c769e78582399d87c320e3
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