Roll src/third_party/skia 6c431d52020c..68aeec07de4a (7 commits) (#8590)

https://skia.googlesource.com/skia.git/%2Blog/6c431d52020c..68aeec07de4a

git log 6c431d52020c6ed8d48681c9443ec3131fd0ed34..68aeec07de4a1894a5cefde405abcb5a49801df6 --date=short --no-merges --format=%ad %ae %s
2019-04-16 bsalomon@google.com Clarify that buffers of type GrGpuBufferType::kXferGpuToCpu are mapped for reading.
2019-04-16 jvanverth@google.com Convert 565 and 4444 bitmaps to 8888 for Mac Metal.
2019-04-15 csmartdalton@google.com Don't flush pipeline state when querying sample locations in GL
2019-04-15 mtklein@google.com just some refactoring for fm_bot.go
2019-04-15 egdaniel@google.com Remove deprecated SkSurface::prepareForExternalIO and related uses of the name.
2019-04-10 bsalomon@google.com GrSurface -> GrGpuBuffer transfer improvements:
2019-04-10 bsalomon@google.com Avoid multiplication by alpha in fragment shader when known to be 1.

The AutoRoll server is located here: https://autoroll.skia.org/r/skia-flutter-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 (herb@google.com), and stop
the roller if necessary.

DEPS

@@ -23,7 +23,7 @@ vars = {
   'fuchsia_git': 'https://fuchsia.googlesource.com',
   'github_git': 'https://github.com',
   'skia_git': 'https://skia.googlesource.com',
-  'skia_revision': '6c431d52020c6ed8d48681c9443ec3131fd0ed34',
+  'skia_revision': '68aeec07de4a1894a5cefde405abcb5a49801df6',
 
   # 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: 92d0339ce145006f8f1f584f35c38078
+Signature: 1f1d4e10470c3d2050811a5a9e0f9e1a
 
 UNUSED LICENSES:
 
@@ -6010,8 +6010,11 @@ FILE: ../../../third_party/skia/src/gpu/GrSamplePatternDictionary.h
 FILE: ../../../third_party/skia/src/gpu/ccpr/GrGSCoverageProcessor.h
 FILE: ../../../third_party/skia/src/gpu/ccpr/GrVSCoverageProcessor.h
 FILE: ../../../third_party/skia/src/gpu/effects/GrMixerEffect.fp
+FILE: ../../../third_party/skia/src/gpu/effects/GrOverrideInputFragmentProcessor.fp
 FILE: ../../../third_party/skia/src/gpu/effects/generated/GrMixerEffect.cpp
 FILE: ../../../third_party/skia/src/gpu/effects/generated/GrMixerEffect.h
+FILE: ../../../third_party/skia/src/gpu/effects/generated/GrOverrideInputFragmentProcessor.cpp
+FILE: ../../../third_party/skia/src/gpu/effects/generated/GrOverrideInputFragmentProcessor.h
 FILE: ../../../third_party/skia/src/gpu/vk/GrVkSecondaryCBDrawContext.cpp
 FILE: ../../../third_party/skia/src/gpu/vk/GrVkSecondaryCBDrawContext.h
 FILE: ../../../third_party/skia/src/sksl/SkSLDefines.h