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

chrome/browser/printing/print_preview_message_handler.h

Index: chrome/browser/printing/print_preview_message_handler.h
===================================================================
--- chrome/browser/printing/print_preview_message_handler.h	(revision 92494)
+++ chrome/browser/printing/print_preview_message_handler.h	(working copy)
@@ -32,11 +32,10 @@
   void OnRequestPrintPreview();
   void OnDidGetPreviewPageCount(int page_count);
   // |page_number| is 0-based.
-  void OnDidPreviewPage(int page_number, bool* cancel);
+  void OnDidPreviewPage(int page_number, int document_cookie);
   void OnPagesReadyForPreview(
       const PrintHostMsg_DidPreviewDocument_Params& params);
   void OnPrintPreviewFailed(int document_cookie);
-  void OnPrintPreviewCancelled(int document_cookie);
 
   DISALLOW_COPY_AND_ASSIGN(PrintPreviewMessageHandler);
 };