Roll external/skia b831ea229..3ff2b2005 (4 commits)

https://skia.googlesource.com/skia.git/+log/b831ea229..3ff2b2005

2018-06-18 brianosman@google.com Revert "Remove old references to SampleApp"
2018-06-18 brianosman@google.com Remove old references to SampleApp
2018-06-18 herb@google.com Use x text skew to position text
2018-06-18 ethannicholas@google.com added GrSkSLFP and converted DitherEffect to use it

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: Ib584b8ccebb08fb8353150416f2b51a7a00c4335
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.