Roll external/skia bc712740b..56f6e8355 (3 commits)

https://skia.googlesource.com/skia.git/+log/bc712740b..56f6e8355

2017-06-02 robertphillips@google.com Roll ANGLE
2017-06-02 reed@google.com default impl of appendStages now calls shaderContext
2017-06-02 robertphillips@google.com Remove ImmediateFlush mode

Test: Presubmit checks will test this change.
Change-Id: I65cf12ba088508bd9b9eb2a303d42523482e76f1