Roll external/skia 712fe73ca..6bba6de94 (1 commits)

https://skia.googlesource.com/skia.git/+log/712fe73ca..6bba6de94

2017-09-25 angle-deps-roller@chromium.org Roll skia/third_party/externals/angle2/ f0fd87d82..457ded9b4 (1 commit)

Test: Presubmit checks will test this change.
Change-Id: I7e0ea3e5485d531c57f6d97a0092e44d3ff5305a