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 0c5d35d6804e5df32fbdcbf86bb400eeef2407bb..1cb8a9bcc17ad00da8506a476d789f194f2385ba 100644
--- a/tools/gn/label.h
+++ b/tools/gn/label.h
@@ -78,6 +78,13 @@ class Label {
     return toolchain_name_ < other.toolchain_name_;
   }
 
+  void swap(Label& other) {
+    std::swap(dir_, other.dir_);
+    std::swap(name_, other.name_);
+    std::swap(toolchain_dir_, other.toolchain_dir_);
+    std::swap(toolchain_name_, other.toolchain_name_);
+  }
+
   // Returns true if the toolchain dir/name of this object matches some
   // other object.
   bool ToolchainsEqual(const Label& other) const {
@@ -116,4 +123,12 @@ inline size_t hash_value(const Label& v) {
 
 }  // namespace BASE_HASH_NAMESPACE
 
+namespace std {
+
+template<> inline void swap(Label& lhs, Label& rhs) {
+  lhs.swap(rhs);
+}
+
+}  // namespace std
+
 #endif  // TOOLS_GN_LABEL_H_