Roll external/skia 76d640d14..311cc6b39 (2 commits)

https://skia.googlesource.com/skia.git/+log/76d640d14..311cc6b39

2017-09-20 jvanverth@google.com Another attempt to fix TSAN
2017-09-20 angle-deps-roller@chromium.org Roll skia/third_party/externals/angle2/ 805d281ae..c8bee3351 (2 commits)

Test: Presubmit checks will test this change.
Change-Id: I934638735d1e51bb24283ee4f7b49a4cb2688424