Roll src/third_party/skia cbeabd969bbe..3794db84e141 (8 commits) (#14532)

https://skia.googlesource.com/skia.git/+log/cbeabd969bbe..3794db84e141

git log cbeabd969bbe..3794db84e141 --date=short --first-parent --format='%ad %ae %s'
2019-12-17 bsalomon@google.com Blend functions as SkSL built ins.
2019-12-17 egdaniel@google.com Add SurfaceView for SkImage_GpuYUVA's flattened proxy.
2019-12-17 brianosman@google.com Don't allocate invalidation messages for generators that make uncacheable textures
2019-12-17 halcanary@google.com Skottie: fix public.bzl
2019-12-17 fmalita@chromium.org Python3 tweaks
2019-12-17 robertphillips@google.com Update SKP version (take 2)
2019-12-17 halcanary@google.com skottie_ios_app: fix tap logic
2019-12-17 robertphillips@google.com Additional DDL fix

Created with:
  gclient setdep -r src/third_party/skia@3794db84e141

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 borenet@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: borenet@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': 'cbeabd969bbec50c399bd68cc8b9f9d4d7f9bf62',
+  'skia_revision': '3794db84e141861052fd5a58e518c70e9a7350a4',
 
   # 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: acfc2d2b7d69fe9ce0260e6c21152c70
+Signature: 906978426b3b0029447310d708640242
 
 UNUSED LICENSES:
 
@@ -1475,6 +1475,7 @@ FILE: ../../../third_party/skia/specs/web-img-decode/proposed/impl/impl.js
 FILE: ../../../third_party/skia/specs/web-img-decode/proposed/index.html
 FILE: ../../../third_party/skia/src/core/SkOrderedReadBuffer.h
 FILE: ../../../third_party/skia/src/sksl/lex/sksl.lex
+FILE: ../../../third_party/skia/src/sksl/sksl_blend.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