Reduce number of allocations in AutoStopwatch

This patch fixes two related issues.
1. The AutoStopwatch uses a stack-allocated `mozilla::Vector` to
communicate with its callback during each compartment switch.
This vector was designed to allow its contents to be stack-allocated
but they turned out to be accidentally heap-allocated.

2. During each tick, the stopwatch fills a vector `recentGroups_`.
This vector always started with minimal capacity and had to grow
repeatedly as groups were added, causing repeated reallocations.
This patch preallocates `recentGroups_` to have the same capacity as the
previous tick. We expect that this should eventually reach a stable size
that closely matches the actual needs of the process.

toolkit/components/perfmonitoring/nsPerformanceStats.cpp

@@ -1082,6 +1082,9 @@ nsPerformanceStatsService::GetPerformanceGroups(JSContext* cx,
     return false;
   }
 
+  // Returning a vector that is too large would cause allocations all over the
+  // place in the JS engine. We want to be sure that all data is stored inline.
+  MOZ_ASSERT(out.length() <= out.sMaxInlineStorage);
   return true;
 }
 
@@ -1310,8 +1313,12 @@ nsPerformanceStatsService::GetResources(uint64_t* userTime,
 
 void
 nsPerformanceStatsService::NotifyJankObservers(const mozilla::Vector<uint64_t>& aPreviousJankLevels) {
-  GroupVector alerts;
-  mPendingAlerts.swap(alerts);
+
+  // The move operation is generally constant time, unless
+  // `mPendingAlerts.length()` is very small, in which case it's fast anyway.
+  GroupVector alerts(Move(mPendingAlerts));
+  mPendingAlerts = GroupVector(); // Reconstruct after `Move`.
+
   if (!mPendingAlertsCollector) {
     // We are shutting down.
     return;