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

tools/gn/script_values.h

Index: tools/gn/script_values.h
diff --git a/tools/gn/script_values.h b/tools/gn/script_values.h
index ac4f0db520fb669d0cf067cd8874a4c76fdb3e8b..50219ef61d0b68425a998f40bcc74242310a89c8 100644
--- a/tools/gn/script_values.h
+++ b/tools/gn/script_values.h
@@ -32,10 +32,16 @@ class ScriptValues {
   const std::vector<SourceFile>& outputs() const { return outputs_; }
   void swap_in_outputs(std::vector<SourceFile>* op) { outputs_.swap(*op); }
 
+  // Depfile generated by the script.
+  const SourceFile& depfile() const { return depfile_; }
+  bool has_depfile() const { return !depfile_.is_null(); }
+  void set_depfile(const SourceFile& depfile) { depfile_ = depfile; }
+
  private:
   SourceFile script_;
   std::vector<std::string> args_;
   std::vector<SourceFile> outputs_;
+  SourceFile depfile_;
 
   DISALLOW_COPY_AND_ASSIGN(ScriptValues);
 };