Roll external/skia 6274c42b8..cc4221be5 (1 commits)

https://skia.googlesource.com/skia.git/+log/6274c42b8..cc4221be5

2017-08-18 benjaminwagner@google.com Rename Mac bots to match reality.

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Change-Id: Ibfbcc15736e0c3c7a2cbdebec37ca0567b15e14b