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Themed system tray background was added at crrev.com/c/2006370 and
removed at crrev.com/c/2398988, since themed background is controlled
inside AshColorProvider after crrev.com/c/2242223.

- Remove the unnecessary logic inside GetBackgroundColor that was added
  to theme the system tray background color.
- Always add blur for system tray.

Then the background color of the system tray will always be
kTransparent80 with 30 blur sigma, based on the latest spec at
https://www.figma.com/file/rQFGy3iAMfNdDnydTGkvHr/Style-%7C-Semantic-color-system-surface-theming?node-id=372%3A0

Bug: 1130618
Change-Id: Ib38a30f6f287baa63a3f2544374224e526859e92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424646Reviewed-by: Tim Song <tengs@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809607}