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 2576f898a13095d7b49b2cb525811cd8a8f849b1..a2f861fbc8ea6e178388053ba8fd9398762aaeba 100644
--- a/Source/core/frame/FrameHost.cpp
+++ b/Source/core/frame/FrameHost.cpp
@@ -49,6 +49,7 @@ FrameHost::FrameHost(Page& page)
     , m_pinchViewport(PinchViewport::create(*this))
     , m_eventHandlerRegistry(adoptPtrWillBeNoop(new EventHandlerRegistry(*this)))
     , m_consoleMessageStorage(ConsoleMessageStorage::createForFrameHost(this))
+    , m_subframeCount(0)
 {
 }
 
@@ -100,4 +101,25 @@ void FrameHost::trace(Visitor* visitor)
     visitor->trace(m_consoleMessageStorage);
 }
 
+#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;
+}
+
 }