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required by media/cast.

The full media target cannot be easily built for some platform (namely
iOS aarch64), typically because of assembly code. The new target has no
such issues.

The change also includes modifications to the media/cast build file to
simplify the dependency list and enable builds using the media_cast
component via a build variable.

BUG=415335

Review URL: https://codereview.chromium.org/581803003

Cr-Commit-Position: refs/heads/master@{#296101}