Roll external/skia 9fc5e8da9804..f813ef7d1b74 (4 commits)

https://skia.googlesource.com/skia.git/+log/9fc5e8da9804..f813ef7d1b74

2019-08-09 senorblanco@chromium.org Dawn backend: implement stencil, primitive topology, separate blends.
2019-08-09 bsalomon@google.com Add SkUNREACHABLE to definition of SK_ABORT.
2019-08-09 bungeman@google.com Move initilization of Lighting sample.
2019-08-09 jlavrova@google.com Build fix

The AutoRoll server is located here: https://skia-autoroll.corp.goog/r/android-master-autoroll

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

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.

Test: Presubmit checks will test this change.
Change-Id: Ie4cf037168663416563ea4b51af301f7b9b7ae1e
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.