Roll external/skia 9194706e8..a8e5744af (3 commits)

https://skia.googlesource.com/skia.git/+log/9194706e8..a8e5744af

2017-09-11 brianosman@google.com Add general three-stop analytic gradient shader
2017-09-11 bsalomon@google.com Remove repeated conditional in TextureOp.
2017-09-11 ethannicholas@google.com Revert "Revert "Switch to the new SkSL lexer.""

Test: Presubmit checks will test this change.
Change-Id: If8bfe8049e2ff1d1dad2ee839fb1fb2f9e08f03b