This changeset modifies the ThreadSafeChromeUtils::saveHeapSnapshot webidl
method to return the path to the core dump file where the heap snapshot was
serialized rather than taking the file path as a parameter.
By removing the ability for callers to choose a path, we pave the way for
enabling taking heap snapshots in sandboxed child processes (e10s, fxos) that do
not have access to the filesystem directly and must be handed a file descriptor
over IPDL. Additionally, the devtools API consumers were not taking advantage of
the ability to choose a file path, and always saving heap snapshots into the
temp directory anyways.
