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

tools/gn/import_manager.h

Index: tools/gn/import_manager.h
diff --git a/tools/gn/import_manager.h b/tools/gn/import_manager.h
index bd47a2ad519989ce6ff93087b60069ea54b200f5..78bb613322af8d8ce867ae6ee7be149d51f1c6f8 100644
--- a/tools/gn/import_manager.h
+++ b/tools/gn/import_manager.h
@@ -31,10 +31,13 @@ class ImportManager {
                 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);