Bug 1529556 - Increase the low-memory detection thresholds by 50% r=njn

After analyzing crash ping data we've established that the current low-memory
detection threshold is too low, there are still a fair number of crashes
happening above it. A 50% increase to 384 MiB should be just about right in
the light of recent telemetry data.

Differential Revision: https://phabricator.services.mozilla.com/D23826

--HG--
extra : moz-landing-system : lando

xpcom/base/AvailableMemoryTracker.cpp

@@ -55,12 +55,12 @@ class nsAvailableMemoryWatcher final : public nsIObserver,
 #  if defined(HAVE_64BIT_BUILD)
   static const size_t kLowVirtualMemoryThreshold = 0;
 #  else
-  static const size_t kLowVirtualMemoryThreshold = 256 * 1024 * 1024;
+  static const size_t kLowVirtualMemoryThreshold = 384 * 1024 * 1024;
 #  endif
 
   // Fire a low-memory notification if we have less than this many bytes of
   // commit space (physical memory plus page file) left.
-  static const size_t kLowCommitSpaceThreshold = 256 * 1024 * 1024;
+  static const size_t kLowCommitSpaceThreshold = 384 * 1024 * 1024;
 
   // Fire a low-memory notification if we have less than this many bytes of
   // physical memory available on the whole machine.