Roll external/skia 17d1896a4..6c8d242b1 (18 commits)

https://skia.googlesource.com/skia.git/+log/17d1896a4..6c8d242b1

2018-09-17 herb@google.com Make atomic lists list for bitmaps and paths
2018-09-17 brianosman@google.com Revert "Add Short4 vertex attributes, and benchmark them as 4.12 colors"
2018-09-17 brianosman@google.com Add Short4 vertex attributes, and benchmark them as 4.12 colors
2018-09-17 bsalomon@google.com When merging into an already chained op update the head's op bounds.
2018-09-17 bungeman@google.com Use onOnceBeforeDraw instead of constructor for glyphs.
2018-09-17 kjlubick@google.com Make fuzz::next overloads more consistent
2018-09-17 skia-autoroll@skia-public.iam.gserviceaccount.com Roll third_party/externals/angle2 017bda429667..a49f60fee74d (3 commits)
2018-09-17 chinmaygarde@google.com Allow users of Skia to optionally link against X11 and GL on Linux.
2018-09-17 mtklein@google.com implement SkPixmap::getPixel() with this->readPixels().
2018-09-17 mtklein@google.com implement SkPixmap::erase() with draws
2018-09-17 jvanverth@google.com Some more shadow cleanup
2018-09-17 mtklein@google.com add a couple bitmap cases to P3 GM
2018-09-17 herb@google.com Fix nullptr deref in cache lookup
2018-09-17 mtklein@google.com erase _every_ row of F32 SkPixmaps
2018-09-17 brianosman@google.com Add half-float vertex attribute caps bit and benchmark
2018-09-17 benjaminwagner@google.com [infra] Marker for IntelBayTrail BIOS update.
2018-09-17 csmartdalton@google.com ccpr: Don't require flat interpolation
2018-09-17 herb@google.com Simplify descriptors and thighten context lifetime

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: Id21a95b58f3f3ce399ab908b14a5963dea7c41c9
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.