Roll src/third_party/skia a268dfb74add..a4b448831cee (10 commits) (#11658)

https://skia.googlesource.com/skia.git/+log/a268dfb74add..a4b448831cee

git log a268dfb74add..a4b448831cee --date=short --no-merges --format='%ad %ae %s'
2019-08-28 michaelludwig@google.com Reland "Calculate draw bounds in drawEdgeAAImageSet"
2019-08-28 michaelludwig@google.com Reland "Combine snapBackImage and snapSpecial"
2019-08-28 recipe-mega-autoroller@chops-service-accounts.iam.gserviceaccount.com Roll recipe dependencies (trivial).
2019-08-28 brianosman@google.com Revert "Pin the GLSL version to be no larger than the GL version"
2019-08-28 michaelludwig@google.com Fix non-zero special-image subsets in magnifier filter
2019-08-28 michaelludwig@google.com Revert "Fix flutter roll"
2019-08-28 scroggo@google.com SkAnimatedImage: Update HEIF duration
2019-08-28 jvanverth@google.com Implement fences and semaphores for Metal.
2019-08-28 brianosman@google.com Pin the GLSL version to be no larger than the GL version
2019-08-28 bsalomon@google.com Alpha8 support cleanup in GrGLCaps.

Created with:
  gclient setdep -r src/third_party/skia@a4b448831cee

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/skia-flutter-autoroll
Please CC stani@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

Bug: None
TBR=stani@google.com

DEPS

@@ -26,7 +26,7 @@ vars = {
   'skia_git': 'https://skia.googlesource.com',
   # OCMock is for testing only so there is no google clone
   'ocmock_git': 'https://github.com/erikdoe/ocmock.git',
-  'skia_revision': 'a268dfb74add0aeb299800b4b2d88ed9c8c1ca4c',
+  'skia_revision': 'a4b448831cee38ea9718031b62eaf87851d7c87e',
 
   # When updating the Dart revision, ensure that all entries that are
   # dependencies of Dart are also updated to match the entries in the

ci/licenses_golden/licenses_skia

-Signature: 0562bd9b68450b2a9eb8492d377f11f8
+Signature: 68c9f0e942ca2405115a6d1181f4ec75
 
 UNUSED LICENSES:
 
@@ -3385,6 +3385,8 @@ FILE: ../../../third_party/skia/src/gpu/effects/generated/GrOverrideInputFragmen
 FILE: ../../../third_party/skia/src/gpu/mtl/GrMtlCommandBuffer.h
 FILE: ../../../third_party/skia/src/gpu/mtl/GrMtlCommandBuffer.mm
 FILE: ../../../third_party/skia/src/gpu/mtl/GrMtlDepthStencil.h
+FILE: ../../../third_party/skia/src/gpu/mtl/GrMtlSemaphore.h
+FILE: ../../../third_party/skia/src/gpu/mtl/GrMtlSemaphore.mm
 FILE: ../../../third_party/skia/src/gpu/vk/GrVkSecondaryCBDrawContext.cpp
 FILE: ../../../third_party/skia/src/gpu/vk/GrVkSecondaryCBDrawContext.h
 FILE: ../../../third_party/skia/src/shaders/SkRTShader.cpp