Bug 1203840 - Trigger dirty pages purge after CC. r=njn,r=smaug,r=mccr8

Jemalloc 4 purges dirty pages regularly during free() when the ratio of dirty
pages compared to active pages is higher than 1 << lg_dirty_mult.  We set
lg_dirty_mult in jemalloc_config to limit RSS usage, but it also has an impact
on performance.

So instead of enforcing a high ratio to force more pages being purged, we keep
jemalloc's default ratio of 8, and force a regular purge of all dirty pages,
after cycle collection.

Keeping jemalloc's default ratio avoids cycle-collection-triggered purge to
have to go through really all dirty pages when there are a lot, in which case
the normal jemalloc purge during free() will already have kicked in. It also
takes care of everything that doesn't run the cycle collector still having
a level of purge, like plugins in the plugin-container.

At the same time, since jemalloc_purge_freed_pages does nothing with jemalloc 4,
repurpose the MEMORY_FREE_PURGED_PAGES_MS telemetry probe to track the time
spent in this cycle-collector-triggered purge.

xpcom/base/nsMemoryReporterManager.cpp

@@ -457,10 +457,12 @@ static nsresult
 ResidentDistinguishedAmountHelper(int64_t* aN, bool aDoPurge)
 {
 #ifdef HAVE_JEMALLOC_STATS
+#ifndef MOZ_JEMALLOC4
   if (aDoPurge) {
     Telemetry::AutoTimer<Telemetry::MEMORY_FREE_PURGED_PAGES_MS> timer;
     jemalloc_purge_freed_pages();
   }
+#endif
 #endif
 
   task_basic_info ti;