Roll external/skia 5b6c7c645..d8f7da46d (5 commits)

https://skia.googlesource.com/skia.git/+log/5b6c7c645..d8f7da46d

2017-08-02 skia-deps-roller@chromium.org Roll skia/third_party/externals/angle2/ df7d7c9ec..8f276e255 (1 commit)
2017-08-02 brianosman@google.com Guard against D3D NaN/Infinity literals bug in color burn
2017-08-02 mtklein@chromium.org Revert "remove another SkConvertPixels "fast path""
2017-08-02 rmistry@google.com Update webpages_playback.py to use the new Golang impl of WPRs
2017-08-02 benjaminwagner@google.com Update all Debian bots to 9.1.

Test: Presubmit checks will test this change.
Change-Id: I16e8081caab2d26357b3f9832a6c06b62cf5d0ad