Add a UniqueVector class to GN

tools/gn/source_file.h

Index: tools/gn/source_file.h
diff --git a/tools/gn/source_file.h b/tools/gn/source_file.h
index 83ce2b59d50bc182e205e1d0bac9fd5d9ed38c5e..3b657beb7ea1a7c6d4816648f2ad53f97adc750b 100644
--- a/tools/gn/source_file.h
+++ b/tools/gn/source_file.h
@@ -5,6 +5,7 @@
 #ifndef TOOLS_GN_SOURCE_FILE_H_
 #define TOOLS_GN_SOURCE_FILE_H_
 
+#include <algorithm>
 #include <string>
 
 #include "base/containers/hash_tables.h"
@@ -69,6 +70,10 @@ class SourceFile {
     return value_ < other.value_;
   }
 
+  void swap(SourceFile& other) {
+    value_.swap(other.value_);
+  }
+
  private:
   friend class SourceDir;
 
@@ -94,4 +99,8 @@ inline size_t hash_value(const SourceFile& v) {
 
 }  // namespace BASE_HASH_NAMESPACE
 
+inline void swap(SourceFile& lhs, SourceFile& rhs) {
+  lhs.swap(rhs);
+}
+
 #endif  // TOOLS_GN_SOURCE_FILE_H_