Reland "Streamline frame detach"

Source/core/frame/FrameHost.cpp

Index: Source/core/frame/FrameHost.cpp
diff --git a/Source/core/frame/FrameHost.cpp b/Source/core/frame/FrameHost.cpp
index 45f7d1cfd3177d19ee2ddf062aca9a06c4214a92..d7fbfd7020835f612e30c2a05fc30499691aa181 100644
--- a/Source/core/frame/FrameHost.cpp
+++ b/Source/core/frame/FrameHost.cpp
@@ -47,6 +47,7 @@ FrameHost::FrameHost(Page& page)
     : m_page(&page)
     , m_pinchViewport(PinchViewport::create(*this))
     , m_eventHandlerRegistry(adoptPtrWillBeNoop(new EventHandlerRegistry(*this)))
+    , m_subframeCount(0)
 {
 }
 
@@ -92,4 +93,25 @@ void FrameHost::trace(Visitor* visitor)
     visitor->trace(m_eventHandlerRegistry);
 }
 
+#if ENABLE(ASSERT)
+void checkFrameCountConsistency(int expectedFrameCount, Frame* frame)
+{
+    ASSERT(expectedFrameCount >= 0);
+
+    int actualFrameCount = 0;
+    for (; frame; frame = frame->tree().traverseNext())
+        ++actualFrameCount;
+
+    ASSERT(expectedFrameCount == actualFrameCount);
+}
+#endif
+
+int FrameHost::subframeCount() const
+{
+#if ENABLE(ASSERT)
+    checkFrameCountConsistency(m_subframeCount + 1, m_page->mainFrame());
+#endif
+    return m_subframeCount;
+}
+
 }