Task Manager should listen to WebContentsObserver::RenderFrameCreated.

content/public/browser/render_frame_host.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_PUBLIC_BROWSER_RENDER_FRAME_HOST_H_
 #define CONTENT_PUBLIC_BROWSER_RENDER_FRAME_HOST_H_
 
 #include <string>
 
 #include "base/callback_forward.h"
@@ skipping 180 matching lines @@
   virtual AssociatedInterfaceProvider* GetRemoteAssociatedInterfaces() = 0;
 
   // Returns the visibility state of the frame. The different visibility states
   // of a frame are defined in Blink.
   virtual blink::WebPageVisibilityState GetVisibilityState() = 0;
 
   // Returns whether the RenderFrame in the renderer process has been created
   // and still has a connection.  This is valid for all frames.
   virtual bool IsRenderFrameLive() = 0;
 
+  // This method returns false only for RenderFrameHost created for the purpose
+  // of swapping out another RenderFrameHost, until the swap actually happens.

[ncarter (slow), 2017/05/16 20:50:57]

// Returns true if this is the currently-visible RenderFrameHost for our frame
tree node. During process transfer, a RenderFrameHost may be created that is not
current. After process transfer, the old RenderFrameHost becomes non-current
until it is deleted (which may not happen until its unload handler runs).
// 
// Changes to the IsCurrent() state of a RenderFrameHost may be observed via
WebContentsObserver::RenderFrameHostChanged().

[Łukasz Anforowicz, 2017/05/16 21:33:44]

Done.

+  virtual bool IsCurrent() = 0;
+
   // Get the number of proxies to this frame, in all processes. Exposed for
   // use by resource metrics.
   virtual int GetProxyCount() = 0;
 
   // Notifies the Listener that one or more files have been chosen by the user
   // from a file chooser dialog for the form. |permissions| is the file
   // selection mode in which the chooser dialog was created.
   virtual void FilesSelectedInChooser(
       const std::vector<content::FileChooserFileInfo>& files,
       FileChooserParams::Mode permissions) = 0;
@@ skipping 44 matching lines @@
 
  private:
   // This interface should only be implemented inside content.
   friend class RenderFrameHostImpl;
   RenderFrameHost() {}
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_RENDER_FRAME_HOST_H_