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

tools/gn/value_extractors.h

 // 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.
 
 #ifndef TOOLS_GN_VALUE_EXTRACTORS_H_
 #define TOOLS_GN_VALUE_EXTRACTORS_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 47 matching lines @@
                          const SourceDir& current_dir,
                          const Label& current_toolchain,
                          LabelConfigVector* dest,
                          Err* err);
 bool ExtractListOfLabels(const Value& value,
                          const SourceDir& current_dir,
                          const Label& current_toolchain,
                          LabelTargetVector* dest,
                          Err* err);
 
+bool ExtractRelativeFile(const BuildSettings* build_settings,
+                         const Value& value,
+                         const SourceDir& current_dir,
+                         SourceFile* file,
+                         Err* err);
+
 #endif  // TOOLS_GN_VALUE_EXTRACTORS_H_