Roll Skia from d15c52d2333e to 6d2b7688f64d (4 revisions)

https://skia.googlesource.com/skia.git/+log/d15c52d2333e..6d2b7688f64d

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: If7c8ad05428d9e5d00e6fec061df946d518ce727