GitLab

In Background class, the default value for |color_| is not used in any
of the subclasses: either a subclass will override the color with
SetNativeControlColor(), or a subclass will not use |color_| at all. So
change the initial value to a placeholder color.

Also replace CreateStandardPanelBackground() with directly calling
CreateThemedSolidBackground() to make shell window to use themed colors
and to remove unnecessary wrapping function.

BUG=1059867

Change-Id: I97c6cf8943046453516ba93dc3b779eb6740d658
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2102785
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753734}