Bug 925416: Report on memory used by blob URLs.

By default, the reporter path includes the blob owner's URI and the
blob URI in the path.  If a blob has multiple references, its size is
divided among them so that the totals still work; the blob's address and
refcount are included in the path (to find the other refs), and all of
this is noted in the description.

If memory.blob_report.stack_frames is a nonzero value, that many JS
stack frames (file and line number, if available) will be added to the
path, with the file root-relative to the owner if same-origin.