Task Manager should listen to WebContentsObserver::RenderFrameCreated.

content/public/browser/render_frame_host.h

Index: content/public/browser/render_frame_host.h
diff --git a/content/public/browser/render_frame_host.h b/content/public/browser/render_frame_host.h
index 72cbc18bbafaea6d07ae5e29f5e9c42fc69c5d0a..715010bf9443b64b66026eb40bc43062fc1978bc 100644
--- a/content/public/browser/render_frame_host.h
+++ b/content/public/browser/render_frame_host.h
@@ -198,6 +198,10 @@ class CONTENT_EXPORT RenderFrameHost : public IPC::Listener,
   // 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;