Roll external/skia 6ad5289ea..1b1b65748 (13 commits)

https://skia.googlesource.com/skia.git/+log/6ad5289ea..1b1b65748

2019-05-03 fmalita@chromium.org Fix .bzl skottie_lib_hdrs
2019-05-03 benjaminwagner@google.com Fixes for experimental/go-skia
2019-05-03 halcanary@google.com docs/examples/Colors
2019-05-03 fmalita@chromium.org Add explicit .bzl sources and headers lists for skottie/sksg
2019-05-03 halcanary@google.com Shaper Tests: fix g3 build
2019-05-03 ethannicholas@google.com SkSL interpreter optimizations
2019-05-03 halcanary@google.com Shaper Tests: make a macro, rename test resources.
2019-05-03 halcanary@google.com SkColors: Fix new color constants
2019-05-03 fmalita@chromium.org [skottie] Relocate "GMs" to resources/
2019-05-03 herb@google.com Reland "SkShaper JSON output with cluster visualization"
2019-05-03 jvanverth@google.com Move GrMtlBufferManager functionality to better places
2019-05-03 halcanary@google.com Shaper : new unit test
2019-05-03 halcanary@google.com SkShaper: new benchmarks

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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.
Change-Id: I05481d1b11d5cf31d94406db9e317ecc9f7b4954
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