Roll external/skia 6bba6de94..f6f7cf609 (1 commits)

https://skia.googlesource.com/skia.git/+log/6bba6de94..f6f7cf609

2017-09-26 brianosman@google.com Invalidate path VBs when paths are destroyed

Test: Presubmit checks will test this change.
Change-Id: I89a8d8a37b021bc61dff929cd72db186b8033064