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);
-
-  // 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_