Roll external/skia 9a9841178078..ffdc3e668061 (1 commits)

https://skia.googlesource.com/skia.git/+log/9a9841178078..ffdc3e668061

2019-09-20 ethannicholas@google.com added support for SkSL unpremul function

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://skia-autoroll.corp.goog/r/android-master-autoroll
Please CC nifong@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

Test: Presubmit checks will test this change.
Change-Id: I79a24ede942671ef0dce5d3285b7852919812870
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.