Implement NavigationThrottle::BLOCK_REQUEST_AND_COLLAPSE.

content/common/frame_messages.h

Index: content/common/frame_messages.h
diff --git a/content/common/frame_messages.h b/content/common/frame_messages.h
index 8592fcf5d18ee7a42f81dfcec0241993ce94e1dc..2c8ae2ed8de85df2a23537327ffe95dd7b531b38 100644
--- a/content/common/frame_messages.h
+++ b/content/common/frame_messages.h
@@ -813,6 +813,11 @@ IPC_MESSAGE_ROUTED1(FrameMsg_SetAccessibilityMode, content::AccessibilityMode)
 // Dispatch a load event in the iframe element containing this frame.
 IPC_MESSAGE_ROUTED0(FrameMsg_DispatchLoad)
 
+// Sent to a subframe to control whether to collapse its the frame owner element
+// in the embedder document, that is, to remove it from the layout as if it did
+// not exist.
+IPC_MESSAGE_ROUTED1(FrameMsg_Collapse, bool /* collapsed */)
+
 // Notifies the frame that its parent has changed the frame's sandbox flags.
 IPC_MESSAGE_ROUTED1(FrameMsg_DidUpdateSandboxFlags, blink::WebSandboxFlags)