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

content/content_child.gypi

 # Copyright 2013 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.
 
 {
   'dependencies': [
     '../base/base.gyp:base',
     '../components/mime_util/mime_util.gyp:mime_util',
     '../components/tracing.gyp:tracing',
     '../components/webcrypto/webcrypto.gyp:webcrypto',
@@ skipping 135 matching lines @@
       'child/npapi/npobject_util.h',
       'child/npapi/npruntime_util.cc',
       'child/npapi/npruntime_util.h',
       'child/npapi/plugin_host.cc',
       'child/npapi/plugin_host.h',
       'child/npapi/plugin_instance.cc',
       'child/npapi/plugin_instance.h',
       'child/npapi/plugin_instance_mac.mm',
       'child/npapi/plugin_lib.cc',
       'child/npapi/plugin_lib.h',
-      'child/npapi/plugin_stream.cc',
-      'child/npapi/plugin_stream.h',
-      'child/npapi/plugin_stream_posix.cc',
-      'child/npapi/plugin_stream_url.cc',
-      'child/npapi/plugin_stream_url.h',
-      'child/npapi/plugin_stream_win.cc',
-      'child/npapi/plugin_string_stream.cc',
-      'child/npapi/plugin_string_stream.h',
-      'child/npapi/plugin_url_fetcher.cc',
-      'child/npapi/plugin_url_fetcher.h',
       'child/npapi/plugin_web_event_converter_mac.h',
       'child/npapi/plugin_web_event_converter_mac.mm',
       'child/npapi/webplugin.h',
       'child/npapi/webplugin_accelerated_surface_mac.h',
       'child/npapi/webplugin_delegate.h',
       'child/npapi/webplugin_delegate_impl.cc',
       'child/npapi/webplugin_delegate_impl.h',
       'child/npapi/webplugin_delegate_impl_android.cc',
       'child/npapi/webplugin_delegate_impl_aura.cc',
       'child/npapi/webplugin_delegate_impl_mac.mm',
@@ skipping 166 matching lines @@
         'child/npapi/webplugin_delegate_impl_mac.mm',
       ],
     }],
     ['OS=="win"', {
       'sources!': [
         'child/npapi/webplugin_delegate_impl_aura.cc',
       ],
     }],
   ],
 }