Use white header for app windows part #3

This CL simplifies the crossfade logic in FrameCaptionButton and makes FrameCaptionButton make use of |CustomButton::hover_animation_| again.

BUG=297187
TEST=None
R=jamescook@chromium.org

Review URL: https://codereview.chromium.org/152323002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253036 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253036