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- It keeps breaking and requires updating.
- So build it by default, the feature is still behind a runtime flag.
- GN files are cleaned up so that "mojo_media_host" is set with
  clearer logic.

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ic78d4c949e0c8d57707f1d7f213c3b2fabd4b689
Reviewed-on: https://chromium-review.googlesource.com/959446
Commit-Queue: Rintaro Kuroiwa <rkuroiwa@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542840}