Roll external/skia c75ab6a53..f975b3eb6 (1 commits)

https://skia.googlesource.com/skia.git/+log/c75ab6a53..f975b3eb6

2017-06-22 halcanary@google.com Revert "Revert "SkRegion: more robust validation""

Test: Presubmit checks will test this change.
Change-Id: I7799a152cd3d2ccaafa71f084431ca8968388cc3