Roll external/skia d8f7da46d..73d8fd906 (3 commits)

https://skia.googlesource.com/skia.git/+log/d8f7da46d..73d8fd906

2017-08-02 stani@google.com Add a private API for writing the clip to the stencil
2017-08-02 ethannicholas@google.com Revert "Revert "support for 'half' types in sksl, plus general numeric type improvements""
2017-08-02 robertphillips@google.com Fix DeferredTextureImage test on Mac

Test: Presubmit checks will test this change.
Change-Id: Ifcd9bc90e8b742fff63cc75a717281841e2796a2