Add DownloadPDF Callback function.

chrome/renderer/plugins/pdf_plugin_placeholder.h

 // Copyright 2017 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_PLUGINS_PDF_PLUGIN_PLACEHOLDER_H_
 #define CHROME_RENDERER_PLUGINS_PDF_PLUGIN_PLACEHOLDER_H_
 
 #include "components/plugins/renderer/plugin_placeholder.h"
 
 // Placeholder that allows users to click to download a PDF for when
@@ skipping 12 matching lines @@
       const blink::WebPluginParams& params);
 
  private:
   PDFPluginPlaceholder(content::RenderFrame* render_frame,
                        const blink::WebPluginParams& params,
                        const std::string& html_data);
   ~PDFPluginPlaceholder() final;
 
   // WebViewPlugin::Delegate methods:
   v8::Local<v8::Value> GetV8Handle(v8::Isolate* isolate) final;
+
+  gin::ObjectTemplateBuilder GetObjectTemplateBuilder(
+      v8::Isolate* isolate) final;
+
+  void DownloadPDFCallback();
 };
 
 #endif  // CHROME_RENDERER_PLUGINS_PDF_PLUGIN_PLACEHOLDER_H_