Print Preview: Make preview generation event driven to eliminate synchronous messages.

chrome/browser/printing/print_preview_message_handler.h

 // Copyright (c) 2011 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_PRINTING_PRINT_PREVIEW_MESSAGE_HANDLER_H_
 #define CHROME_BROWSER_PRINTING_PRINT_PREVIEW_MESSAGE_HANDLER_H_
 #pragma once
 
 #include "content/browser/tab_contents/tab_contents_observer.h"
 
@@ skipping 12 matching lines @@
   // TabContentsObserver implementation.
   virtual bool OnMessageReceived(const IPC::Message& message);
   virtual void DidStartLoading();
 
  private:
   // Gets the print preview tab associated with |owner_|.
   TabContents* GetPrintPreviewTab();
 
   // Message handlers.
   void OnRequestPrintPreview();
-  void OnDidGetPreviewPageCount(int page_count);
+  void OnDidGetPreviewPageCount(int document_cookie, int page_count);
   // |page_number| is 0-based.
-  void OnDidPreviewPage(int page_number, bool* cancel);
+  void OnDidPreviewPage(int page_number);
   void OnPagesReadyForPreview(
       const PrintHostMsg_DidPreviewDocument_Params& params);
   void OnPrintPreviewFailed(int document_cookie);
-  void OnPrintPreviewCancelled(int document_cookie);
 
   DISALLOW_COPY_AND_ASSIGN(PrintPreviewMessageHandler);
 };
 
 }  // namespace printing
 
 #endif  // CHROME_BROWSER_PRINTING_PRINT_PREVIEW_MESSAGE_HANDLER_H_