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

tools/gn/value_extractors.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/value_extractors.h"
 
 #include "tools/gn/build_settings.h"
 #include "tools/gn/err.h"
 #include "tools/gn/label.h"
 #include "tools/gn/source_dir.h"
@@ skipping 95 matching lines @@
 }
 
 bool ExtractListOfLabels(const Value& value,
                          const SourceDir& current_dir,
                          const Label& current_toolchain,
                          std::vector<Label>* dest,
                          Err* err) {
   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);
+}