Roll external/skia a8316554c..024072af6 (1 commits)

https://skia.googlesource.com/skia.git/+log/a8316554c..024072af6

2018-11-11 mtklein@google.com Reland "Improve degenerate 2pt conical gradient cases"

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Change-Id: I7a8fe6b79bcf5326f09bb12ca72e95ab44b6335e
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