Roll external/skia 6c8d242b1..29a49bd22 (2 commits)

https://skia.googlesource.com/skia.git/+log/6c8d242b1..29a49bd22

2018-09-17 mtklein@google.com Revert "implement SkPixmap::getPixel() with this->readPixels()."
2018-09-17 mtklein@google.com simplify src->dst matrix init

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Change-Id: Iea0f1907248823cc39f09682c7884f0f37d90077
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