Bug 973090 - IPC remoting for child GC/CC logging. r=mccr8 r=???

This has a few semi-interdependent pieces:

* Factoring out the file opening/closing/renaming from the GC/CC logging.

* Using IPC to have the child log to files that the parent opened.

* Changing nsIMemoryInfoDumper.dumpGCAndCCLogsToFile to report completion
  of child process logging (which was impossible before this, and which is
  needed to have a meaningful test case).

* Changing about:memory to dump logs for child processes, matching the
  behavior of the "Measure" button, because it can tell the user where
  they are now.

* Add a test for multiprocess GC/CC log dumping (only of the XPCOM
  interface, not by clicking buttons and scraping the about:memory page,
  but as a chrome mochitest to start remote browsers).