This commit creates the HeapSnapshotFileActor and moves the transferHeapSnapshot
method from MemoryActor to HeapSnapshotFileActor. This is necessary because
child processes in e10s are sandboxed and do not have access to the file system,
and because MemoryActor is in the sandboxed child process it cannot open the
file and send it over the RDP.
This complexity is hidden at the MemoryFront layer. Users of MemoryFront will
still simply call its saveHeapSnapshot method, and do not need to worry about
the inner workings of how heap snapshot files are transferred over the RDP. This
required adding a third parameter to MemoryFront's initialize method: the
listTabs response.
