Eliminate superfluous clearing of weak pointer.

third_party/WebKit/Source/core/dom/IntersectionObservation.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/IntersectionObservation.h"
 
 #include "core/dom/ElementRareData.h"
 #include "core/dom/IntersectionObserver.h"
 #include "core/frame/FrameView.h"
 #include "core/layout/LayoutBox.h"
@@ skipping 98 matching lines @@
     if (geometry.intersectionRect.size().isZero())
         geometry.intersectionRect = LayoutRect();
     if (!m_shouldReportRootBounds)
         geometry.rootRect = LayoutRect();
 
     return true;
 }
 
 void IntersectionObservation::computeIntersectionObservations(double timestamp)
 {
+    // Pre-oilpan, there will be a delay between the time when the target Element gets deleted
+    // (because its ref count dropped to zero) and when this IntersectionObservation gets
+    // deleted (during the next gc run, because the target Element is the only thing keeping
+    // the IntersectionObservation alive).  During that interval, we need to check that m_target
+    // hasn't been cleared.
     Element* targetElement = target();
     if (!targetElement || !isActive())
         return;
     LayoutObject* targetLayoutObject = targetElement->layoutObject();
     // TODO(szager): Support SVG
     if (!targetLayoutObject || (!targetLayoutObject->isBox() && !targetLayoutObject->isInline()))
         return;
 
     IntersectionGeometry geometry;
     if (!computeGeometry(geometry))
@@ skipping 25 matching lines @@
     m_observer.clear();
 }
 
 DEFINE_TRACE(IntersectionObservation)
 {
     visitor->trace(m_observer);
     visitor->trace(m_target);
 }
 
 } // namespace blink