Roll external/skia 21d3f9016..1798739ab (1 commits)

https://skia.googlesource.com/skia.git/+log/21d3f9016..1798739ab

2018-04-13 angle-skia-autoroll@skia-buildbots.google.com.iam.gserviceaccount.com Roll third_party/externals/angle2/ cc1293775..983c429fa (1 commit)

The AutoRoll server is located here: https://android-roll.skia.org

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: I262fabdb59196c765004191e1beb8fcb6fb909c0
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.