Roll external/skia 4a09815d8e98..ad21d47cfa8d (4 commits)

https://skia.googlesource.com/skia.git/+log/4a09815d8e98..ad21d47cfa8d

2019-11-11 recipe-mega-autoroller@chops-service-accounts.iam.gserviceaccount.com Roll recipe dependencies (trivial).
2019-11-11 bsalomon@google.com Clamp RGB outputs of GrYUVtoRGBEffect.
2019-11-11 herb@google.com Remove belongsToCache from SkStrike
2019-11-11 jlavrova@google.com Correct cluster index

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: I56ee46dd8c63409e21e2a1e09b7c87fb67739ea1
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.