GN: Add ability to specify a depfile for custom targets.

tools/gn/value_extractors.cc

Index: tools/gn/value_extractors.cc
diff --git a/tools/gn/value_extractors.cc b/tools/gn/value_extractors.cc
index c2ca28527429d92d144192a66f21edf06bd3290b..3d8809f4f4a42afa35eb729831a29961ae52e4af 100644
--- a/tools/gn/value_extractors.cc
+++ b/tools/gn/value_extractors.cc
@@ -113,3 +113,11 @@ bool ExtractListOfLabels(const Value& value,
   return ListValueExtractor(value, dest, err,
                             LabelResolver(current_dir, current_toolchain));
 }
+
+bool ExtractRelativeFile(const BuildSettings* build_settings,
+                         const Value& value,
+                         const SourceDir& current_dir,
+                         SourceFile* file, Err* err) {
+  RelativeFileConverter converter(build_settings, current_dir);
+  return converter(value, file, err);
+}