Roll external/skia 1400d38e0..30cc00c64 (3 commits)

https://skia.googlesource.com/skia.git/+log/1400d38e0..30cc00c64

2018-08-03 kjlubick@google.com Add job for building PathKit to WASM using Docker
2018-08-03 timliang@google.com validate GrMtlBuffer after forcing it to be static
2018-08-03 brianosman@google.com Avoid coverage bleed for AA arcs with tiny sweep angles

The AutoRoll server is located here: https://android-master-roll.skia.org

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
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Bug: 112018278
Change-Id: I68f7c43cbfb204ef8828b86fa74371697e79565f