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This CL stores the background_color manifest field in the WebApp
database for use by experimental tabbed web app windows.
This CL has no user visible changes.

Bug: 1104083
Change-Id: I69ad67c17b61161e721a4639340187550e0644b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2291515Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788814}