Roll Skia from 6d2b7688f64d to b21bc6e8e926 (6 revisions)

https://skia.googlesource.com/skia.git/+log/6d2b7688f64d..b21bc6e8e926

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 djsollen@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/+doc/master/autoroll/README.md

Test: Presubmit checks will test this change.
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.
Change-Id: I30871111d3b1a64dfda454d594105f8258aa5d70