Bug 1460385 - Don't trace propid in TraceCycleCollectorChildren. r=sfink

The propid is a jsid, and a jsid can never be anything the CC cares
about, so there's no reason to trace it. In at least one profile of
Google Inbox, tracing shape propids is taking the majority of time in
the CC and causing very long pauses.

MozReview-Commit-ID: HcjnawBHLx1

--HG--
extra : rebase_source : 3682b2ac84752b200be780401059c2907dc7d6af

js/src/gc/Tracer.cpp

@@ -194,7 +194,7 @@ gc::TraceCycleCollectorChildren(JS::CallbackTracer* trc, Shape* shape)
         MOZ_ASSERT(shape->base());
         shape->base()->assertConsistency();
 
-        TraceEdge(trc, &shape->propidRef(), "propid");
+        // Don't trace the propid because the CC doesn't care about jsid.
 
         if (shape->hasGetterObject()) {
             JSObject* tmp = shape->getterObject();