Avoid using declarations in headers which introduce symbol conflicts.

third_party/WebKit/Source/modules/webdatabase/InspectorDatabaseAgent.cpp

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 36 matching lines @@
 #include "modules/webdatabase/SQLTransactionErrorCallback.h"
 #include "modules/webdatabase/sqlite/SQLValue.h"
 #include "wtf/RefCounted.h"
 #include "wtf/Vector.h"
 
 typedef blink::protocol::Database::Backend::ExecuteSQLCallback
     ExecuteSQLCallback;
 
 namespace blink {
 
+using protocol::Maybe;
+using protocol::Response;
+
 namespace DatabaseAgentState {
 static const char databaseAgentEnabled[] = "databaseAgentEnabled";
 };
 
 namespace {
 
 class ExecuteSQLCallbackWrapper : public RefCounted<ExecuteSQLCallbackWrapper> {
  public:
   static PassRefPtr<ExecuteSQLCallbackWrapper> create(
       std::unique_ptr<ExecuteSQLCallback> callback) {
@@ skipping 294 matching lines @@
   return it->value->database();
 }
 
 DEFINE_TRACE(InspectorDatabaseAgent) {
   visitor->trace(m_page);
   visitor->trace(m_resources);
   InspectorBaseAgent::trace(visitor);
 }
 
 }  // namespace blink