Roll external/skia a47c48ef6bce..da037b85352e (3 commits)

https://skia.googlesource.com/skia.git/+log/a47c48ef6bce..da037b85352e

2019-08-12 skia-autoroll@skia-public.iam.gserviceaccount.com Roll third_party/externals/angle2 eb0479e245f0..b90755ba7793 (12 commits)
2019-08-12 skia-recreate-skps@skia-swarming-bots.iam.gserviceaccount.com Update Go deps
2019-08-12 skia-autoroll@skia-public.iam.gserviceaccount.com Roll ../src d9af708ac0cc..186422eefc88 (460 commits)

The AutoRoll server is located here: https://skia-autoroll.corp.goog/r/android-master-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, who should
be CC'd on the roll, and stop the roller if necessary.

Test: Presubmit checks will test this change.
Change-Id: Ib675d17290941360ab621af02405211a1d5d57ba
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.