Makes the current page dim out when transitioning to instant preview.

chrome/browser/instant/instant_delegate.h

 // Copyright (c) 2010 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 CHROME_BROWSER_INSTANT_INSTANT_DELEGATE_H_
 #define CHROME_BROWSER_INSTANT_INSTANT_DELEGATE_H_
 #pragma once
 
 #include "base/string16.h"
 
 class TabContents;
 
 namespace gfx {
 class Rect;
 }
 
 // InstantController's delegate. Normally the Browser implements this. See
 // InstantController for details.
 class InstantDelegate {
  public:
+  // Invoked when instant starts loading, but before the preview tab contents is
+  // ready to be shown. This may be used to animate between the states.
+  // This is followed by ShowInstant and/or HideInstant.
+  virtual void PrepareForInstant() = 0;
+
   // Invoked when the instant TabContents should be shown.
   virtual void ShowInstant(TabContents* preview_contents) = 0;
 
   // Invoked when the instant TabContents should be hidden.
   virtual void HideInstant() = 0;
 
   // Invoked when the user does something that should result in the preview
   // TabContents becoming the active TabContents. The delegate takes ownership
   // of the supplied TabContents.
   virtual void CommitInstant(TabContents* preview_contents) = 0;
 
   // Invoked when the suggested text is to change to |text|.
   virtual void SetSuggestedText(const string16& text) = 0;
 
   // Returns the bounds instant will be placed at in screen coordinates.
   virtual gfx::Rect GetInstantBounds() = 0;
 
  protected:
   virtual ~InstantDelegate() {}
 };
 
 #endif  // CHROME_BROWSER_INSTANT_INSTANT_DELEGATE_H_