Use registerWeakMembers to manage observer root.

third_party/WebKit/Source/core/dom/IntersectionObservation.h

Index: third_party/WebKit/Source/core/dom/IntersectionObservation.h
diff --git a/third_party/WebKit/Source/core/dom/IntersectionObservation.h b/third_party/WebKit/Source/core/dom/IntersectionObservation.h
index 7299922fc5ef8ac4000459652958b7e409c4460f..9388af98ee677bdec9be7d6433dc07a2a9506635 100644
--- a/third_party/WebKit/Source/core/dom/IntersectionObservation.h
+++ b/third_party/WebKit/Source/core/dom/IntersectionObservation.h
@@ -32,9 +32,9 @@ public:
     unsigned lastThresholdIndex() const { return m_lastThresholdIndex; }
     void setLastThresholdIndex(unsigned index) { m_lastThresholdIndex = index; }
     bool shouldReportRootBounds() const { return m_shouldReportRootBounds; }
-
     void computeIntersectionObservations(double timestamp);
     void disconnect();
+    void clearRootAndRemoveFromTarget();
 
     DECLARE_TRACE();