IntersectionObserver: remove gc-timing-revealing execeptions.

third_party/WebKit/Source/core/dom/ElementIntersectionObserverData.cpp

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/dom/ElementIntersectionObserverData.h"
 
 #include "core/dom/Document.h"
 #include "core/dom/IntersectionObservation.h"
 #include "core/dom/IntersectionObserver.h"
 #include "core/dom/IntersectionObserverController.h"
@@ skipping 15 matching lines @@
   m_intersectionObservers.add(&observer);
 }
 
 void ElementIntersectionObserverData::removeObserver(
     IntersectionObserver& observer) {
   m_intersectionObservers.remove(&observer);
 }
 
 void ElementIntersectionObserverData::addObservation(
     IntersectionObservation& observation) {
+  DCHECK(observation.observer());
   m_intersectionObservations.add(
-      TraceWrapperMember<IntersectionObserver>(this, &observation.observer()),
+      TraceWrapperMember<IntersectionObserver>(this, observation.observer()),
       &observation);
 }
 
 void ElementIntersectionObserverData::removeObservation(
     IntersectionObserver& observer) {
   m_intersectionObservations.remove(&observer);
 }
 
 void ElementIntersectionObserverData::activateValidIntersectionObservers(
     Node& node) {
@@ skipping 21 matching lines @@
   visitor->trace(m_intersectionObservations);
 }
 
 DEFINE_TRACE_WRAPPERS(ElementIntersectionObserverData) {
   for (auto& entry : m_intersectionObservations) {
     visitor->traceWrappers(entry.key);
   }
 }
 
 }  // namespace blink