Roll src/third_party/skia 6344cbd1df76..18531cde2ab7 (2 commits) (flutter#12358)

https://skia.googlesource.com/skia.git/+log/6344cbd1df76..18531cde2ab7

git log 6344cbd1df76..18531cde2ab7 --date=short --no-merges --format='%ad %ae %s'
2019-09-19 [email protected] Roll third_party/externals/angle2 a7003d5fc306..bf4268a3963b (7 commits)
2019-09-19 [email protected] Update Go deps

Created with:
  gclient setdep -r src/third_party/skia@18531cde2ab7

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 [email protected] 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
[email protected]

Author: skia-flutter-autoroll

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': '6344cbd1df7634b4d583fd6a2878428ec331c026',
+  'skia_revision': '18531cde2ab757d82d3934fbac6c698e7b53cb3b',
 
   # 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

@@ -1,4 +1,4 @@
-Signature: 91097f982b4ec09ec3cf41fe74e4b315
+Signature: 31cdf3333929b8a9d78e94861aed27f0
 
 UNUSED LICENSES: