Cleanup remaining class/struct fwd declarations

sandbox/linux/bpf_dsl/bpf_dsl_impl.h

 // Copyright 2014 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 SANDBOX_LINUX_BPF_DSL_BPF_DSL_IMPL_H_
 #define SANDBOX_LINUX_BPF_DSL_BPF_DSL_IMPL_H_
 
 #include <memory>
 
 #include "base/macros.h"
 #include "sandbox/linux/bpf_dsl/codegen.h"
 #include "sandbox/sandbox_export.h"
 
 namespace sandbox {
 namespace bpf_dsl {
-class ErrorCode;
 class PolicyCompiler;
 
 namespace internal {
 
 // Internal interface implemented by BoolExpr implementations.
 class BoolExprImpl {
  public:
   // Compile uses |pc| to emit a CodeGen::Node that conditionally continues
   // to either |then_node| or |false_node|, depending on whether the represented
   // boolean expression is true or false.
@@ skipping 32 matching lines @@
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ResultExprImpl);
 };
 
 }  // namespace internal
 }  // namespace bpf_dsl
 }  // namespace sandbox
 
 #endif  // SANDBOX_LINUX_BPF_DSL_BPF_DSL_IMPL_H_