Files.app: Introduce ProgressCenterItemElement class.


Currently the state of item elements of the progress center is managed by the
ProgressCenter class.  But it is getting more complex, so this CL introduces
ProgressCenterItemElement class to manage the state.

BUG=330641
TEST=manually

Review URL: https://codereview.chromium.org/106583005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242514 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242514