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Only one interstitial (of media remoting and picture in picture) will
be shown at a time. This change renames IDs for elements in the
interstitial element with the same styling.

This change also pulls out the CSS into a separate file, per TODO.

Expected further work here includes some more styling updates with
an interstitial icon for picture in picture.

BUG: 805516
Change-Id: I6c2151f2166b4dbbf2aaa0be83f8492369008a11
Reviewed-on: https://chromium-review.googlesource.com/890025Reviewed-by: Mounir Lamouri (slow) <mlamouri@chromium.org>
Reviewed-by: Xiangjun Zhang <xjz@chromium.org>
Commit-Queue: apacible <apacible@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532917}