Roll external/skia 0aa492d0f..9b9510a71 (2 commits)

https://skia.googlesource.com/skia.git/+log/0aa492d0f..9b9510a71

2017-07-18 halcanary@google.com Revert "Revert "SkPDF: Non-outline glyphs as images""
2017-07-18 bsalomon@google.com Make dither range symmetric about 0

Test: Presubmit checks will test this change.
Change-Id: I752d41cdcd0d5f84e94266f9b9e3460f630d8026