Cleanup: Remove unneeded forward declarations from various chrome subdirectories.

chrome/default_plugin/plugin_impl_gtk.h

Index: chrome/default_plugin/plugin_impl_gtk.h
===================================================================
--- chrome/default_plugin/plugin_impl_gtk.h	(revision 109585)
+++ chrome/default_plugin/plugin_impl_gtk.h	(working copy)
@@ -1,4 +1,4 @@
-// Copyright (c) 2010 The Chromium Authors. All rights reserved.
+// Copyright (c) 2011 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.
 
@@ -28,9 +28,6 @@
   PluginInstallerLaunchFailure
 };
 
-class PluginInstallDialog;
-class PluginDatabaseHandler;
-
 // Provides the plugin installation functionality. This class is
 // instantiated with the information like the mime type of the
 // target plugin, the display mode, etc.