Roll external/skia f47abb487..dcdabf3b3 (3 commits)

https://skia.googlesource.com/skia.git/+log/f47abb487..dcdabf3b3

2018-11-13 herb@google.com Move strike cache backpointer to glyph cache
2018-11-13 skia-autoroll@skia-public.iam.gserviceaccount.com Roll third_party/externals/swiftshader 1147fc2a5dac..6ed398b46847 (1 commits)
2018-11-13 mtklein@google.com drop sub_y == 0 optimization

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: Ia401d169fa8864aa63cbdec3568fcccd7b022fe8
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.