Roll external/skia 9a725dd94..f20e77bcb (3 commits)

https://skia.googlesource.com/skia.git/+log/9a725dd94..f20e77bcb

2017-09-20 angle-deps-roller@chromium.org Roll skia/third_party/externals/angle2/ 47bf2dc53..729b2c6e0 (1 commit)
2017-09-20 benjaminwagner@google.com Fix float-cast-overflow in SkTypeface_FreeType::onGetAdvancedMetrics
2017-09-20 bsalomon@google.com Extra safety for SkRRect.

Test: Presubmit checks will test this change.
Change-Id: I1f48aa76560ca18597333bd6f6f3e9985c50cb4a