Files.app: Remove the ButterBar class.

BatterBar is replaced with new ProgressCenter class. This is no longer
needed. This CL removes the ButterBar class, HTML elements and CSS styles.

BUG=318931
TEST=manually
R=yoshiki@chromium.org

Review URL: https://codereview.chromium.org/98213003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238333 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238333