System dialog does not need any additional changing of page size.

chrome/renderer/printing/print_web_view_helper.h

 // Copyright (c) 2012 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_RENDERER_PRINTING_PRINT_WEB_VIEW_HELPER_H_
 #define CHROME_RENDERER_PRINTING_PRINT_WEB_VIEW_HELPER_H_
 
 #include <vector>
 
 #include "base/gtest_prod_util.h"
@@ skipping 193 matching lines @@
 #if defined(OS_LINUX) || defined(OS_ANDROID)
   void PrintPageInternal(const PrintMsg_PrintPage_Params& params,
                          const gfx::Size& canvas_size,
                          blink::WebFrame* frame,
                          Metafile* metafile);
 #elif defined(WIN_PDF_METAFILE_FOR_PRINTING)
   void PrintPageInternal(const PrintMsg_PrintPage_Params& params,
                          const gfx::Size& canvas_size,
                          blink::WebFrame* frame,
                          Metafile* metafile,
-                         bool is_preview,
                          gfx::Size* page_size_in_dpi,
                          gfx::Rect* content_area_in_dpi);
 #else
   void PrintPageInternal(const PrintMsg_PrintPage_Params& params,
                          const gfx::Size& canvas_size,
                          blink::WebFrame* frame);
 #endif
 
   // Render the frame for printing.
   bool RenderPagesForPrint(blink::WebLocalFrame* frame,
@@ skipping 231 matching lines @@
   PrintPreviewContext print_preview_context_;
   bool is_loading_;
   bool is_scripted_preview_delayed_;
   base::WeakPtrFactory<PrintWebViewHelper> weak_ptr_factory_;
   DISALLOW_COPY_AND_ASSIGN(PrintWebViewHelper);
 };
 
 }  // namespace printing
 
 #endif  // CHROME_RENDERER_PRINTING_PRINT_WEB_VIEW_HELPER_H_