Remove obviously unneeded forward declarations from headers under src/{app,base,net}.

net/base/sdch_filter.h

Index: net/base/sdch_filter.h
diff --git a/net/base/sdch_filter.h b/net/base/sdch_filter.h
index 4c96f49e7c46ec42eb7779530c83153359eefa72..963cf326cf32b309f87b588e96903e327ba9a863 100644
--- a/net/base/sdch_filter.h
+++ b/net/base/sdch_filter.h
@@ -1,4 +1,4 @@
-// Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
+// Copyright (c) 2010 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.
 
@@ -21,8 +21,6 @@
 #include "net/base/filter.h"
 #include "net/base/sdch_manager.h"
 
-class SafeOutputStringInterface;
-
 namespace open_vcdiff {
   class VCDiffStreamingDecoder;
 }