Bug 1378970 - Part 1: Label CollectorRunner::mScheduleTimer using SystemGroup. r=billm

All other associated timers in this file use the SystemGroup for a
GarbageCollection task, and this one should too.

MozReview-Commit-ID: GjdAGxbZS03

dom/base/nsJSEnvironment.cpp

@@ -352,6 +352,7 @@ public:
           if (!mScheduleTimer) {
             return;
           }
+          mScheduleTimer->SetTarget(SystemGroup::EventTargetFor(TaskCategory::GarbageCollection));
         } else {
           mScheduleTimer->Cancel();
         }