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

tools/gn/script_target_generator.h

Index: tools/gn/script_target_generator.h
diff --git a/tools/gn/script_target_generator.h b/tools/gn/script_target_generator.h
index 593dd01bad21061b63874d1579a44760710fd6c9..f693145a060482b4f23f0529b5e63facd9a98bbb 100644
--- a/tools/gn/script_target_generator.h
+++ b/tools/gn/script_target_generator.h
@@ -12,9 +12,9 @@
 class ScriptTargetGenerator : public TargetGenerator {
  public:
   ScriptTargetGenerator(Target* target,
-                      Scope* scope,
-                      const Token& function_token,
-                      Err* err);
+                        Scope* scope,
+                        const Token& function_token,
+                        Err* err);
   virtual ~ScriptTargetGenerator();
 
  protected:
@@ -23,6 +23,7 @@ class ScriptTargetGenerator : public TargetGenerator {
  private:
   void FillScript();
   void FillScriptArgs();
+  void FillDepfile();
 
   DISALLOW_COPY_AND_ASSIGN(ScriptTargetGenerator);
 };