Roll external/skia dfcaaa94b..0221e8b22 (4 commits)

https://skia.googlesource.com/skia.git/+log/dfcaaa94b..0221e8b22

2019-05-13 robertphillips@google.com Revert "Remove some uses of GrBytesPerPixel"
2019-05-13 robertphillips@google.com Remove some uses of GrBytesPerPixel
2019-05-13 mtklein@google.com rewrite docs to explicitly use python2
2019-05-13 brianosman@google.com Interpreter: Fix bool literals

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Change-Id: Ifbc1cc724e5a1aab04144abd28057dc25cd6471b
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