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 done as a chrome mochitest to start remote browsers); based on
test_memoryReporters2.xul in the same directory.
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