Bug 296512 Change DOMGCCallback to *always* chain to other callbacks

r=jst sr=brendan a=asa

dom/src/base/nsJSEnvironment.cpp

@@ -2026,11 +2026,11 @@ nsJSContext::FireGCTimer()
 static JSBool JS_DLL_CALLBACK
 DOMGCCallback(JSContext *cx, JSGCStatus status)
 {
+  JSBool result = gOldJSGCCallback ? gOldJSGCCallback(cx, status) : JS_TRUE;
+
   if (status == JSGC_BEGIN && PR_GetCurrentThread() != gDOMThread)
     return JS_FALSE;
 
-  JSBool result = gOldJSGCCallback ? gOldJSGCCallback(cx, status) : JS_TRUE;
-
   // XPCJSRuntime::GCCallback does marking from the JSGC_MARK_END callback.
   // we need to call EndGCMark *after* marking is finished.
   // XXX This relies on our callback being registered after