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This CL changes MediaNotificationBackground to use the native theme for
determining the default background color instead of using a constant
white color. This fixes an issue where the GMC dialog was white even
when dark mode was on.

Bug: 981218
Change-Id: I668fe7af83286242e4fb5505b49acf53b185af79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775372Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691665}