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This arg is almost enabled on all platforms (except Fuchsia) and
actually controls very few files. Most of the mojo media related
features are controlled by subsidiary buildflags like ENABLE_MOJO_CDM.

This CL removes this arg to make the code cleaner.

Bug: 1067786
Change-Id: I49cb56afefc08902734ab861c87cbbf98cc6366f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2133851Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756799}