Bug 1427525, try to not leak IdleTaskRunners related to GC/CC during shutdow, r=mccr8

dom/base/nsJSEnvironment.cpp

@@ -1636,6 +1636,10 @@ nsJSContext::BeginCycleCollectionCallback()
 
   MOZ_ASSERT(!sICCRunner, "Tried to create a new ICC timer when one already existed.");
 
+  if (sShuttingDown) {
+    return;
+  }
+
   // Create an ICC timer even if ICC is globally disabled, because we could be manually triggering
   // an incremental collection, and we want to be sure to finish it.
   sICCRunner = IdleTaskRunner::Create(ICCRunnerFired,
@@ -1863,6 +1867,11 @@ void
 GCTimerFired(nsITimer *aTimer, void *aClosure)
 {
   nsJSContext::KillGCTimer();
+  nsJSContext::KillInterSliceGCRunner();
+  if (sShuttingDown) {
+    return;
+  }
+
   // Now start the actual GC after initial timer has fired.
   sInterSliceGCRunner = IdleTaskRunner::Create([aClosure](TimeStamp aDeadline) {
     return InterSliceGCRunnerFired(aDeadline, aClosure);