Roll external/skia 19b674c5d..27c8404d8 (2 commits)

https://skia.googlesource.com/skia.git/+log/19b674c5d..27c8404d8

2018-09-19 halcanary@google.com IWYU: three files
2018-09-19 bsalomon@google.com Revert "Revert "Reland "Add ability to specify different GP textures for each mesh in a draw."""

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Change-Id: Ia8fc2fc0da2804a7adea6f6e24c5cc7df8ce5531
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