Add a UniqueVector class to GN

tools/gn/label.h

Index: tools/gn/label.h
diff --git a/tools/gn/label.h b/tools/gn/label.h
index 2b7ef4e9a5cadb77118406fda4267ce035caa782..1028b3088c85f2c38141d173a17f9e62b2301684 100644
--- a/tools/gn/label.h
+++ b/tools/gn/label.h
@@ -83,6 +83,13 @@ class Label {
     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 {
@@ -121,4 +128,8 @@ inline size_t hash_value(const Label& v) {
 
 }  // namespace BASE_HASH_NAMESPACE
 
+inline void swap(Label& lhs, Label& rhs) {
+  lhs.swap(rhs);
+}
+
 #endif  // TOOLS_GN_LABEL_H_