Roll external/skia 27c8404d8..4a3f5c814 (6 commits)

https://skia.googlesource.com/skia.git/+log/27c8404d8..4a3f5c814

2018-09-19 brianosman@google.com Remove hacks for UShort2 texture coords
2018-09-19 mtklein@google.com always optimize third_party code
2018-09-19 halcanary@google.com test,tools: whitespace changes for clang-format
2018-09-19 mtklein@google.com update clang_linux to Clang 7
2018-09-19 halcanary@google.com include: whitespace changes for clang-format
2018-09-19 mtklein@google.com Revert "Reland "implement SkPixmap::getPixel() with this->readPixels().""

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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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Test: Presubmit checks will test this change.
Change-Id: I994a530d8dd7700b3c95000da1e030317a2ddbde
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