Roll external/skia e9188850e..474d68791 (12 commits)

https://skia.googlesource.com/skia.git/+log/e9188850e..474d68791

2017-12-14 jvanverth@google.com Send TextBlobCache purge messages only to owning cache.
2017-12-14 pirama@google.com Turn on PGO for skia_nanobench, skia_dm
2017-12-14 benjaminwagner@google.com Enable Goma fallback by default.
2017-12-14 benjaminwagner@google.com Switch Goma test jobs to ANGLE/Vulkan.
2017-12-14 angle-deps-roller@chromium.org Roll skia/third_party/externals/angle2/ 9af765dd2..9d4d7f06f (1 commit)
2017-12-14 brianosman@google.com Remove SkClampRange (unused)
2017-12-14 mtklein@chromium.org quick-reject before transforming images
2017-12-14 brianosman@google.com Remove SkColorSpace_Base::MakeNamed, along with most uses of Adobe RGB
2017-12-14 reed@google.com convert pipe over to serial procs
2017-12-14 caryclark@skia.org fix broken flatten example and update usingBookmaker
2017-12-14 reed@google.com pre-api change, preparing for expanding metadata
2017-12-14 angle-deps-roller@chromium.org Roll skia/third_party/externals/angle2/ 21f67ae06..9af765dd2 (1 commit)

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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.
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Test: Build skia_nanobench with ANDROID_PGO_INSTRUMENT=skia
Change-Id: I3e70a2a8778a68f1c755ea8745a0d71236a539ee