Roll external/skia 8c4e70d37..dfcb14a34 (4 commits)

https://skia.googlesource.com/skia.git/+log/8c4e70d37..dfcb14a34

2017-08-24 angle-deps-roller@chromium.org Roll skia/third_party/externals/angle2/ c1346fba6..335d67b88 (3 commits)
2017-08-24 bsalomon@google.com Revert "Threaded generation of software paths"
2017-08-24 bungeman@google.com Make SkTFitsIn turn into less code.
2017-08-24 egdaniel@google.com Fix Metal build from recent command buffer change

Test: Presubmit checks will test this change.
Change-Id: Iea5cef28631c3215f50e71f4f9c0a55107278bfa