Roll src/third_party/skia 602df41025ea..6b797fec83f0 (19 commits) (#8494)

https://skia.googlesource.com/skia.git/%2Blog/602df41025ea..6b797fec83f0

git log 602df41025eadb33e872d0314c340d526c740183..6b797fec83f05cd1c57e0d5505a0f69b9f753c78 --date=short --no-merges --format=%ad %ae %s
2019-04-08 brianosman@google.com Simplify persistent cache code, make GLSL easier for tools to access
2019-04-08 csmartdalton@google.com Add a mustWriteToFragColor() workaround flag
2019-04-08 bsalomon@google.com Start on supporting transfer from GrSurface.
2019-04-08 reed@google.com add flag to stage API change
2019-04-08 herb@google.com Do not use sub-pixel positioning for source space
2019-04-08 reed@google.com extend guard to makeAsALocalMatrixShader for google3
2019-04-08 halcanary@google.com SkPDF: device to use SkTCopyOnFirstWrite<SkPaint>
2019-04-08 nifong@google.com add SkpFilePlayer to externs
2019-04-08 reed@google.com add makeShader() to SkBitmap, and deprecate SkShader version
2019-04-08 herb@google.com Handle translating blob to origin in bulk
2019-04-08 robertphillips@google.com Skip flush in GrDrawingManager if the specifed proxy doesn't have any work
2019-04-08 kjlubick@google.com Initialize unfilled textures based on GrCaps
2019-04-08 michaelludwig@google.com Reland "Support clip masks for SaveLayerRec and drawSpecial in Ganesh"
2019-04-08 ethannicholas@google.com Revert "Reland "Revert "Reland "GPU support for SkMixers""""
2019-04-08 reed@google.com Revert "Revert "Move localmatrix routines to shaderbase, no need to be public.""
2019-04-08 ethannicholas@google.com fixed ArithmeticImageFilter input type
2019-04-07 nigeltao@google.com Update Wuffs version
2019-04-08 mtklein@google.com fm_bot: look for and explain too-late flags
2019-04-08 reed@google.com Reland "Revert "Move localmatrix routines to shaderbase, no need to be public.""

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 (egdaniel@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': '602df41025eadb33e872d0314c340d526c740183',
+  'skia_revision': '6b797fec83f05cd1c57e0d5505a0f69b9f753c78',
 
   # 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: c57817ef7ecddfa3a3e3a4fb74a7e125
+Signature: 4d703499240d61f71bfebdfc7445f256
 
 UNUSED LICENSES:
 
@@ -2683,6 +2683,7 @@ FILE: ../../../third_party/skia/src/sksl/sksl.inc
 FILE: ../../../third_party/skia/src/sksl/sksl_fp.inc
 FILE: ../../../third_party/skia/src/sksl/sksl_frag.inc
 FILE: ../../../third_party/skia/src/sksl/sksl_geom.inc
+FILE: ../../../third_party/skia/src/sksl/sksl_mixer.inc
 FILE: ../../../third_party/skia/src/sksl/sksl_pipeline.inc
 FILE: ../../../third_party/skia/src/sksl/sksl_vert.inc
 ----------------------------------------------------------------------------------------------------
@@ -5784,6 +5785,7 @@ FILE: ../../../third_party/skia/docs/examples/Text_Skew_X.cpp
 FILE: ../../../third_party/skia/docs/examples/Typeface_Methods.cpp
 FILE: ../../../third_party/skia/docs/examples/Xor.cpp
 FILE: ../../../third_party/skia/gm/crbug_918512.cpp
+FILE: ../../../third_party/skia/src/gpu/GrPersistentCacheUtils.h
 ----------------------------------------------------------------------------------------------------
 Copyright 2019 Google LLC.