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Adds a flag (--cast-app-background-color) that changes the background
color used when displaying web apps. This color is often shown briefly
before Chromium renders a web app.

Bug: internal b/70181260
Test: use flag on ATV
Change-Id: I3bc0989aeabbf639116c1b30fffb11f66b56c4cd
Reviewed-on: https://chromium-review.googlesource.com/897466
Commit-Queue: Thoren Paulson <thoren@chromium.org>
Reviewed-by: Stephen Lanham <slan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540340}