Add a UniqueVector class to GN

tools/gn/source_dir.h

Index: tools/gn/source_dir.h
diff --git a/tools/gn/source_dir.h b/tools/gn/source_dir.h
index 99be5c37ec7b12e556cc6968e0ab9d36364e4b7d..719579b15ede8f07eb0fdbd248dbb3ef6e6012ed 100644
--- a/tools/gn/source_dir.h
+++ b/tools/gn/source_dir.h
@@ -5,6 +5,7 @@
 #ifndef TOOLS_GN_SOURCE_DIR_H_
 #define TOOLS_GN_SOURCE_DIR_H_
 
+#include <algorithm>
 #include <string>
 
 #include "base/containers/hash_tables.h"
@@ -90,6 +91,10 @@ class SourceDir {
     return value_ < other.value_;
   }
 
+  void swap(SourceDir& other) {
+    value_.swap(other.value_);
+  }
+
  private:
   friend class SourceFile;
   std::string value_;
@@ -114,4 +119,8 @@ inline size_t hash_value(const SourceDir& v) {
 
 }  // namespace BASE_HASH_NAMESPACE
 
+inline void swap(SourceDir& lhs, SourceDir& rhs) {
+  lhs.swap(rhs);
+}
+
 #endif  // TOOLS_GN_SOURCE_DIR_H_