Delete more probably unused plugin code.

third_party/WebKit/public/web/WebPlugin.h

 /*
  * Copyright (C) 2009, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2014 Opera Software ASA. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 74 matching lines @@
     // Once this method has been called, container() must return nullptr.
     virtual void destroy() = 0;
 
     // Returns the container that this plugin has been initialized with.
     // Must return nullptr if this plugin is scheduled for deletion.
     //
     // NOTE: This container doesn't necessarily own this plugin. For example,
     // if the container has been assigned a new plugin, then the container will
     // own the new plugin, not this old plugin.
     virtual WebPluginContainer* container() const { return nullptr; }
-    virtual void containerDidDetachFromParent() { }
 
     virtual v8::Local<v8::Object> v8ScriptableObject(v8::Isolate*) { return v8::Local<v8::Object>(); }
 
     virtual bool supportsKeyboardFocus() const { return false; }
     virtual bool supportsEditCommands() const { return false; }
     // Returns true if this plugin supports input method, which implements
     // setComposition() and confirmComposition() below.
     virtual bool supportsInputMethod() const { return false; }
 
     virtual bool canProcessDrag() const { return false; }
@@ skipping 82 matching lines @@
 
     virtual bool isPlaceholder() { return true; }
 
 protected:
     ~WebPlugin() { }
 };
 
 } // namespace blink
 
 #endif