GN: Don't import a file more than once.

tools/gn/import_manager.h

 // Copyright (c) 2013 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_GN_IMPORT_MANAGER_H_
 #define TOOLS_GN_IMPORT_MANAGER_H_
 
 #include <map>
 #include <memory>
 
@@ skipping 13 matching lines @@
   ~ImportManager();
 
   // Does an import of the given file into the given scope. On error, sets the
   // error and returns false.
   bool DoImport(const SourceFile& file,
                 const ParseNode* node_for_err,
                 Scope* scope,
                 Err* err);
 
  private:
-  base::Lock lock_;
+  struct ImportInfo;
+
+  // Protects access to imports_. Do not hold when actually executing imports.
+  base::Lock imports_lock_;
 
   // Owning pointers to the scopes.
-  typedef std::map<SourceFile, std::unique_ptr<const Scope>> ImportMap;
+  typedef std::map<SourceFile, std::unique_ptr<ImportInfo>> ImportMap;
   ImportMap imports_;
 
   DISALLOW_COPY_AND_ASSIGN(ImportManager);
 };
 
 #endif  // TOOLS_GN_IMPORT_MANAGER_H_