Use a better highlight images for "hover" state and pushed "state".

BUG=18452
TEST=Try different themes and make sure the extension toolstrips have a nice hover and "pushed" images (by hovering over and then clicking and holding the left mouse button).

Review URL: http://codereview.chromium.org/201125

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@26287 0039d316-1c4b-4281-b951-d872f2087c98