Roll external/skia 912cd3d2e..6c1a7f2c7 (4 commits)

https://skia.googlesource.com/skia.git/+log/912cd3d2e..6c1a7f2c7

2018-09-19 skia-autoroll@skia-public.iam.gserviceaccount.com Roll third_party/externals/angle2 5c72f28d1a3f..d511948b9f82 (6 commits)
2018-09-19 bungeman@google.com SkImageGeneratorCG.h to only expose factory method.
2018-09-19 michaelludwig@google.com Increase precision for two point gradient coordinates.
2018-09-19 halcanary@google.com SkPDF: draw persepctive text

The AutoRoll server is located here: https://autoroll-internal.skia.org/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: Ie1f8d76ebec6443e2b55bc289805128d3348c050
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.