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//services/viz/public/mojom:mojom_blink is multiply included when
building blink_unittests.

  A) it's built into the blink_unittests executable
  B) it's built into the blink_platform component, which is a dep of
     blink_unittests

This multiple inclusion lead to double definition errors[1] when I tried
to add a new test[2] in blink_unittests.

The test uses media/mojo/mojom/video_decode_perf_history.mojom-blink.h,
generated by //media/mojo/mojom:mojom_blink.

//media/mojo/mojom:mojom_blink is *already* a dep of blink_unittests via
this chain through blink_platform (component)

  //third_party/blink/renderer/controller:blink_unittests
  //third_party/blink/renderer/controller:blink_unittests_sources
  //third_party/blink/renderer/platform:platform
  //third_party/blink/renderer/platform:blink_platform_public_deps
  //third_party/blink/public/mojom:mojom_platform_blink
  //media/mojo/mojom:mojom_blink

//media/mojo/mojom:mojom_blink is *also* a dep of
//services/viz/public/mojom:mojom_blink, which is built directly into
blink_unittests as a source_set.

  //third_party/blink/renderer/controller:blink_unittests
  //third_party/blink/renderer/controller:blink_unittests_sources
  //third_party/blink/renderer/modules:unit_tests
  //services/viz/public/mojom:mojom_blink
  //media/mojo/mojom:mojom_blink

This explains the error[1]

FAILED: blink_unittests.exe
ninja -t msvc ...
lld-link: error: duplicate symbol: public: __thiscall
media::mojom::blink::PredictionFeatures::PredictionFeatures(void)
>defined at obj/media/mojo/mojom/mojom_blink/media_types.mojom-blink.obj
>defined at blink_platform.dll

Hence, this CL removes multiple inclusion by deleting rules that cause
//services/viz/public/mojom:mojom_blink to be directly built into the
blink_unittests executable.

The blink_platform component is now the sole provider of
//services/viz/public/mojom:mojom_blink to blink_unittests.

This also required small export_* additions to the viz BUILD.gn to give
the viz mojom symbols visibility outside the blink_platform component.
Those changes were also made to gpu's BUILD.gn because blink_unittests
had come to rely on the viz dependency to provide unexported gpu
symbols.

[1] https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8886718386427952624/+/steps/compile__with_patch_/0/stdout
[2] https://chromium-review.googlesource.com/c/chromium/src/+/2088494

Change-Id: I1a7cd262f09a08c532479e44572f50360a6ebc40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088493Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
Auto-Submit: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747568}