Roll external/skia 47238c79c..5bb4c0a42 (2 commits)

https://skia.googlesource.com/skia.git/+log/47238c79c..5bb4c0a42

2017-08-10 skia-deps-roller@chromium.org Roll skia/third_party/externals/angle2/ 0f7714ecc..553590a57 (1 commit)
2017-08-10 brianosman@google.com Revert "Revert "GrContext::dump that produces JSON formatted output""

Test: Presubmit checks will test this change.
Change-Id: Id5d51763f17d28f1a69ea143c28d1a54de485834