Swap MD and non-MD colors for detached bookmark bar background.
Swap MD and non-MD colors for detached bookmark bar background.

This undoes what looks like an accidental change in
https://codereview.chromium.org/1526813002/ , where the old code in
search_ui.cc used white for MD and #f1f1f1 for non-MD, but the new code in
theme_properties.cc swapped these.

Since then, kDefaultDetachedBookmarkBarBackgroundIncognito was added, which
copied the white non-MD background, so this sets that to #f1f1f1 too.

BUG=none
TEST=none
TBR=pkotwicz

Review URL: https://codereview.chromium.org/1581933008

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#370208}

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