Roll external/skia 6b955167286d..20b177a9bf2a (4 commits)

https://skia.googlesource.com/skia.git/+log/6b955167286d..20b177a9bf2a

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: Ifc94367b08d85dd7bda777556bb5450e259779e8
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.