Roll external/skia 21a7be074..42e7a7ed6 (1 commits)

https://skia.googlesource.com/skia.git/+log/21a7be074..42e7a7ed6

2018-12-14 reed@google.com use SkFont for text

The AutoRoll server is located here: https://autoroll-internal.skia.org/r/android-master-autoroll

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.

Test: Presubmit checks will test this change.
Change-Id: I53dcd89aefd6726a4c980b1420c4122af048c2b7
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.