Files.app: Match the progress center UI with the mock.

This CL updates HTML and CSS in order to match the progress center UI with the
mock.

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

Review URL: https://codereview.chromium.org/25680002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226691 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226691