Roll external/skia eb64b9b20..91a1053f2 (1 commits)

https://skia.googlesource.com/skia.git/+log/eb64b9b20..91a1053f2

2017-06-22 ethannicholas@google.com Revert "Revert "clean up sksl dead variable handling""

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Change-Id: I93359e98a520b5fa5de91cc171921c870e93cd85