Clear opener when frame is detached.

third_party/WebKit/Source/web/OpenedFrameTracker.cpp

Index: third_party/WebKit/Source/web/OpenedFrameTracker.cpp
diff --git a/third_party/WebKit/Source/web/OpenedFrameTracker.cpp b/third_party/WebKit/Source/web/OpenedFrameTracker.cpp
index cc96b6440528143a05845bd3935c15318213a4e7..30c44ba10f4b1901cbd9304e3ea202308faaa763 100644
--- a/third_party/WebKit/Source/web/OpenedFrameTracker.cpp
+++ b/third_party/WebKit/Source/web/OpenedFrameTracker.cpp
@@ -11,7 +11,9 @@ namespace blink {
 
 OpenedFrameTracker::OpenedFrameTracker() {}
 
-OpenedFrameTracker::~OpenedFrameTracker() {}
+OpenedFrameTracker::~OpenedFrameTracker() {
+  DCHECK(isEmpty());
+}
 
 bool OpenedFrameTracker::isEmpty() const {
   return m_openedFrames.isEmpty();
@@ -33,9 +35,4 @@ void OpenedFrameTracker::transferTo(WebFrame* opener) {
     frame->setOpener(opener);
 }
 
-void OpenedFrameTracker::traceFrames(Visitor* visitor) {
-  for (WebFrame* frame : m_openedFrames)
-    WebFrame::traceFrame(visitor, frame);
-}
-
 }  // namespace blink