Cleanup remaining class/struct fwd declarations

tools/clang/blink_gc_plugin/CheckFieldsVisitor.h

 // Copyright 2015 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 TOOLS_BLINK_GC_PLUGIN_CHECK_FIELDS_VISITOR_H_
 #define TOOLS_BLINK_GC_PLUGIN_CHECK_FIELDS_VISITOR_H_
 
 #include <vector>
 
 #include "Edge.h"
 
-struct BlinkGCPluginOptions;
 class FieldPoint;
 
 // This visitor checks that the fields of a class are "well formed".
 // - OwnPtr and RefPtr must not point to a GC derived type.
 // - Part objects must not be a GC derived type.
 // - An on-heap class must never contain GC roots.
 // - Only stack-allocated types may point to stack-allocated types.
 
 class CheckFieldsVisitor : public RecursiveEdgeVisitor {
  public:
@@ skipping 24 matching lines @@
  private:
   Error InvalidSmartPtr(Edge* ptr);
 
   FieldPoint* current_;
   bool stack_allocated_host_;
   bool managed_host_;
   Errors invalid_fields_;
 };
 
 #endif  // TOOLS_BLINK_GC_PLUGIN_CHECK_FIELDS_VISITOR_H_