Roll external/skia f51e1fd37d90..9a9841178078 (4 commits)

https://skia.googlesource.com/skia.git/+log/f51e1fd37d90..9a9841178078

2019-09-20 skia-autoroll@skia-public.iam.gserviceaccount.com Roll third_party/externals/angle2 bf4268a3963b..6c4ff5ddab07 (8 commits)
2019-09-20 skia-recreate-skps@skia-swarming-bots.iam.gserviceaccount.com Update Go deps
2019-09-20 skia-autoroll@skia-public.iam.gserviceaccount.com Roll ../src b8149cbe28ad..3243c3b11405 (453 commits)
2019-09-20 skia-autoroll@skia-public.iam.gserviceaccount.com Roll third_party/externals/swiftshader e9b6f28fdd71..b6fa949c4539 (2 commits)

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://skia-autoroll.corp.goog/r/android-master-autoroll
Please CC nifong@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

Test: Presubmit checks will test this change.
Change-Id: I7c7b70f8880ca5b754e007eaf60a4d04755a879f
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.