Roll external/skia d20cf3347..6f59443c9 (2 commits)

https://skia.googlesource.com/skia.git/+log/d20cf3347..6f59443c9

2019-01-31 bsalomon@google.com Update Metal unit test blacklist after adding new promise image tests
2019-01-31 bsalomon@google.com Add option to skip intermediate release/fulfill calls for promise images.

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
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Test: Presubmit checks will test this change.
Change-Id: I15bb619c8cbb47c23e96ad17e72fc0bca5c62ac8
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.