Bug 937766, part 5 - Implement PrepareForGarbageCollection. r=smaug

Running the garbage collector can cause objects in the CC graph to
die, so just finish off an incremental cycle collection when we
start a GC.

xpcom/base/CycleCollectedJSRuntime.cpp

@@ -1109,6 +1109,7 @@ CycleCollectedJSRuntime::OnGC(JSGCStatus aStatus)
 {
   switch (aStatus) {
     case JSGC_BEGIN:
+      nsCycleCollector_prepareForGarbageCollection();
       break;
     case JSGC_END:
     {