Bug 1589345 - Fix a leftover comment to tell what PrioritizedEventQueue::mActive actually does, r=farre

EnsureIsActive is called in two places, one is for non-idle tasks, the other for idle tasks
https://searchfox.org/mozilla-central/rev/97976753a21c1731e18177de9e5ce78ea3b3da2d/xpcom/threads/PrioritizedEventQueue.cpp#212-214,286,288,303

Differential Revision: https://phabricator.services.mozilla.com/D49592

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

xpcom/threads/PrioritizedEventQueue.h

@@ -198,7 +198,7 @@ class PrioritizedEventQueue final : public AbstractEventQueue {
   RefPtr<ipc::IdleSchedulerChild> mIdleScheduler;
   bool mIdleSchedulerInitialized = false;
 
-  // mActive tells whether the event queue is running non-idle tasks.
+  // mActive tells whether the event queue is running tasks.
   bool mActive = true;
 };