Implement file-wise cancel for copy/move on cross file system.

This CL implements file-wise cancellation for copy and move operation on cross
file system.
Note that cancellation of Copy/Move on a same file system isn't supported,
assuming the operation is quickly done usually.

BUG=279287
TEST=Ran unit_tests and content_unittests
R=kinuko@chromium.org

Review URL: https://codereview.chromium.org/24625002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225668 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225668