Add a UniqueVector class to GN

tools/gn/label.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_LABEL_H_
 #define TOOLS_GN_LABEL_H_
 
 #include "base/containers/hash_tables.h"
 #include "build/build_config.h"
 #include "tools/gn/source_dir.h"
@@ skipping 65 matching lines @@
     // (one for operator==, one for <).
     if (dir_ != other.dir_)
       return dir_ < other.dir_;
     if (name_ != other.name_)
       return name_ < other.name_;
     if (toolchain_dir_ != other.toolchain_dir_)
       return toolchain_dir_ < other.toolchain_dir_;
     return toolchain_name_ < other.toolchain_name_;
   }
 
+  void swap(Label& other) {
+    dir_.swap(other.dir_);
+    name_.swap(other.name_);
+    toolchain_dir_.swap(other.toolchain_dir_);
+    toolchain_name_.swap(other.toolchain_name_);
+  }
+
   // Returns true if the toolchain dir/name of this object matches some
   // other object.
   bool ToolchainsEqual(const Label& other) const {
     return toolchain_dir_ == other.toolchain_dir_ &&
            toolchain_name_ == other.toolchain_name_;
   }
 
  private:
   SourceDir dir_;
   std::string name_;
@@ skipping 18 matching lines @@
 inline size_t hash_value(const Label& v) {
   return ((hash_value(v.dir().value()) * 131 +
            hash_value(v.name())) * 131 +
           hash_value(v.toolchain_dir().value())) * 131 +
          hash_value(v.toolchain_name());
 }
 #endif  // COMPILER...
 
 }  // namespace BASE_HASH_NAMESPACE
 
+inline void swap(Label& lhs, Label& rhs) {
+  lhs.swap(rhs);
+}
+
 #endif  // TOOLS_GN_LABEL_H_