Roll external/skia b3c0bf642..0c6daf034 (8 commits)

https://skia.googlesource.com/skia.git/+log/b3c0bf642..0c6daf034

2019-05-17 robertphillips@google.com Direct backend object allocation API
2019-05-16 brianosman@google.com Remove Nima from DEPS, experiments, Viewer, etc.
2019-05-16 jvanverth@google.com Revise Mac windowing and context creation.
2019-05-16 brianosman@google.com Interpreter: Change all Load/Store ops to use immediate indices
2019-05-16 robertphillips@google.com Make createTestingOnlyBackendTexture and deleteTestingOnlyBackendTexture no longer be behind GR_TEST_UTILS (take 2)
2019-05-16 skia-autoroll@skia-public.iam.gserviceaccount.com Roll ../src a01010a50904..8c742b264faa (455 commits)
2019-05-16 skia-autoroll@skia-public.iam.gserviceaccount.com Roll third_party/externals/angle2 bff32703c75b..6ddb7768fa25 (10 commits)
2019-05-16 skia-autoroll@skia-public.iam.gserviceaccount.com Roll third_party/externals/swiftshader 3d7b7ea134d1..d61595f367af (10 commits)

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Change-Id: I810c1565d20cda0fae116dae43ad4862913b6e40
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