bug 1007965

* DownloadHelper should use new 'storageName' and 'storagePath' properties on DOM Download object instead
  of munging path manually.
* DownloadStore should save 'storageName' and 'storagePath' properties to DataStore for later use.

r=crdlc
Merge pull request #19454 from nullaus/bug1007965

bug 1007965