Files.app: Implement the cancel logic for the new progress center.

Previously the new progress center had the view of canceled item but it did not
have the logic to cancel the item.  This CL adds actual cancel logic to the new
progress center.

BUG=277432
TEST=manually

Review URL: https://codereview.chromium.org/26671003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228115 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228115