Roll external/skia 4dc5a4540..969be1c98 (6 commits)
https://skia.googlesource.com/skia.git/+log/4dc5a4540..969be1c98
2018-05-21 bsalomon@google.com Remove mask-filters and aa from SkPaint in SkCanvas for nine-patch/lattice.
2018-05-21 benjaminwagner@google.com Omit failing test.
2018-05-21 enne@chromium.org Fix unit tests when workarounds are enabled
2018-05-21 egdaniel@google.com Reland "Remove GrBackendObject and all related functions from Skia."
2018-05-21 bsalomon@google.com Add perspective support to GrTextureOp.
2018-05-21 egdaniel@google.com Move gn setup for vulkan library/headers into their own third_party directory.
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Change-Id: Idccc3baf089598b0d677aaea42894a08f8087079
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