Remove PluginLoadObserver and related logic, it was only used for NPAPI.

content/child/npapi/plugin_string_stream.h

 // Copyright (c) 2006-2008 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 CONTENT_CHILD_NPAPI_PLUGIN_STRING_STREAM_H_
 #define CONTENT_CHILD_NPAPI_PLUGIN_STRING_STREAM_H_
 
 #include "content/child/npapi/plugin_stream.h"
 
 class GURL;
 
 namespace content {
 
 class PluginInstance;
 
 // An NPAPI stream from a string.
 class PluginStringStream : public PluginStream {
  public:
   // Create a new stream for sending to the plugin.
   // If notify_needed, will notify the plugin after the data has
   // all been sent.
   PluginStringStream(PluginInstance* instance,
-                     const GURL& url,
-                     bool notify_needed,
-                     void* notify_data);
+                     const GURL& url);
 
   // Initiates the sending of data to the plugin.
   void SendToPlugin(const std::string& data,
                     const std::string& mime_type);
 
  private:
   ~PluginStringStream() override;
 
   DISALLOW_COPY_AND_ASSIGN(PluginStringStream);
 };
 
 
 }  // namespace content
 
 #endif  // CONTENT_CHILD_NPAPI_PLUGIN_STRING_STREAM_H_