Don't fill combo boxes with the warning color; only the border.

BUG=326017
R=estade@chromium.org, pkasting@chromium.org, sadrul@chromium.org

Review URL: https://codereview.chromium.org/107343006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242846 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242846