Roll external/skia afa35022d..1d6281d4b (3 commits)

https://skia.googlesource.com/skia.git/+log/afa35022d..1d6281d4b

2018-09-18 mtklein@google.com Reland "implement SkPixmap::getPixel() with this->readPixels()."
2018-09-18 fmalita@chromium.org [skottie] Fix NaN |t| in CompTimeMapper::onTick
2018-09-18 fmalita@chromium.org [skottie] UBSAN divide-by-zero workaround

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Change-Id: Ia3bdd1071632004c5823a5a98481879735549919
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