Files.app: Updates the message of progress center.

Previously the progress center was showing fake messages. This CL replace with
the fake messages with the real string assets.

Along the way, the CL removes unused string assets used by the butter bar.

BUG=277432
TEST=manually
R=yoshiki@chromium.org

Review URL: https://codereview.chromium.org/54093002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232665 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232665