Roll external/skia 3928ff8e0..ce4b6c100 (4 commits)

https://skia.googlesource.com/skia.git/+log/3928ff8e0..ce4b6c100

2017-06-06 mtklein@chromium.org less naive lowp constant_color
2017-06-06 mtklein@chromium.org define HWCAP_* ourselves in SkCpu.cpp
2017-06-06 fmalita@chromium.org Add color space support to 4f gradients
2017-06-06 reed@google.com move Coeff into blendmode

Test: Presubmit checks will test this change.
Change-Id: I87b06522a8b9f2ec2e364ef351cd24f249d25703