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

[dcheng, 2016/01/11 22:29:42]

It's not accurate to imply that an Element and any IntersectionObservations
targeting it will be deleted in the same GC cycle. Given how complicated this
comment would be though, I'd suggest just leaving it: we can better document how
we avoid calling this on an observation waiting to be swept with the Oilpan
fixes.

+    // (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.

[haraken, 2016/01/12 00:01:37]

Nit: This comment looks nice (so I want to keep it) but this is an already
widely-used programming pattern for weak pointers. We need to have a null check
before using a weak pointer unless it is guaranteed that the weak pointer must
not be null in some other way.

     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