Roll external/skia eb99bfd85..50e66d4ab (4 commits)

https://skia.googlesource.com/skia.git/+log/eb99bfd85..50e66d4ab

2017-05-16 bsalomon@google.com Remove GrRenderTargetContext::desc()
2017-05-16 egdaniel@google.com Require copy as draws for vulkan cross context image support
2017-05-16 mtklein@chromium.org clamp to premul in dither
2017-05-16 brianosman@google.com Remove Windows Intel blacklisting of cross context texture

Test: Presubmit checks will test this change.
Change-Id: Id224e65426e37079e91a4026b25f89f024634ace