Remove unused ImageButton::SetOverlayImage()

BUG=None
TEST=None
R=jamescook, sadrul
TBR=mukai (For trivial refactor of ui/message_center/views/padded_button.cc)

Review URL: https://codereview.chromium.org/137423008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247489 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247489