Fix comments for the bgcolors indexed by control state in WebTestThemeControlWin.cpp

R=jamesr@chromium.org
BUG=229207

Review URL: https://codereview.chromium.org/61043002

git-svn-id: svn://svn.chromium.org/blink/trunk@161403 bbb929c8-8fbe-4397-9dbb-9b2b20218538