Roll Skia from 512ff75a0bad to b21fac24817a (2 revisions)

https://skia.googlesource.com/skia.git/+log/512ff75a0bad..b21fac24817a

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 stani@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: Ib0a0f02de11556b942657828c1364447d5c4f40c