This changes the way nsMemoryReporterManger handles child processes;
instead of using an observer message and trying to keep a count of child
processes expected to answer, it directly iterates a copy of the list
of content processes and explicitly handles children which exit before
their reports start.
Note that GC/CC logs still run at full concurrency, and that no child
reports start until the parent is finished (see bug 1151597) regardless
of concurrency limit.
92f69b3201