Files.app: Let FileOperatonManager refer files by using Entry.

Previously FileOperationManager uses URL to refer files.
This CL replaces the urls with entries.
Along the way, this CL also removes usage of PathUtil.

BUG=325039
TEST=manually

Review URL: https://codereview.chromium.org/100903004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238674 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238674