Make printing work better with OOPIF.

content/public/renderer/render_frame_observer.h

Index: content/public/renderer/render_frame_observer.h
diff --git a/content/public/renderer/render_frame_observer.h b/content/public/renderer/render_frame_observer.h
index 26a01b1af85d09c0e77ce6df79457567d7c3cba8..7b03e59f01c07fcc173d285b48d340bec13b80cf 100644
--- a/content/public/renderer/render_frame_observer.h
+++ b/content/public/renderer/render_frame_observer.h
@@ -121,6 +121,9 @@ class CONTENT_EXPORT RenderFrameObserver : public IPC::Listener,
   // Called when accessibility is enabled or disabled.
   virtual void AccessibilityModeChanged() {}
 
+  // For printing.

[nasko, 2016/11/14 20:28:52]

nit: Can you add a bit more explanation than "for printing". If I understand it
right, this is the way we actually invoke printing as it is implemented outside
of RF and RF only serves as a conduit for the signal that we need to start a
print, right?

[Lei Zhang, 2016/11/14 23:52:48]

Yes, thank you for reminding me to put it in a better comment. I copied parts of
printPage() from third_party/WebKit/public/web/WebViewClient.h.

+  virtual void ScriptedPrint(bool user_initiated) {}
+
   // IPC::Listener implementation.
   bool OnMessageReceived(const IPC::Message& message) override;