Roll src/third_party/skia a43e2d284b3b..15ecdf94fd36 (6 commits) (#11794)

https://skia.googlesource.com/skia.git/+log/a43e2d284b3b..15ecdf94fd36

git log a43e2d284b3b..15ecdf94fd36 --date=short --no-merges --format='%ad %ae %s'
2019-08-30 egdaniel@google.com Just skip uninstantiated GrOps instead of delete them on flush.
2019-08-30 brianosman@google.com Pass SkPaint by const-ref to SkParticleDrawable
2019-08-30 csmartdalton@google.com Reland "Initiate MSAA resolves during DAG generation"
2019-08-30 ethannicholas@google.com .fp files now support user-defined functions
2019-08-30 ethannicholas@google.com removed dead code from SkSLPipelineStateCodeGenerator
2019-08-30 reed@google.com add bot to test savelayer legacy flag

Created with:
  gclient setdep -r src/third_party/skia@15ecdf94fd36

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': 'a43e2d284b3b9c9cbe689ef5bc093b4d9954d604',
+  'skia_revision': '15ecdf94fd36110285e25351dfa40acc158d60d3',
 
   # 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: 6e8d31a1ee7314c039cad1d4aba7991a
+Signature: 253d1c938c8b6fc4ece856a10850b081
 
 UNUSED LICENSES: