Add a simple one shot and repeating timer API for Mojo.js.

gin/function_template.h

Index: gin/function_template.h
diff --git a/gin/function_template.h b/gin/function_template.h
index 202fe6fb88096982c3ee0193bf5c0095ba877dac..31f67fa3b3d97bff6b147b2002e761956e429b4b 100644
--- a/gin/function_template.h
+++ b/gin/function_template.h
@@ -270,6 +270,13 @@ inline bool GetNextArgument(Arguments* args, int create_flags, bool is_first,
   return true;
 }
 
+// It's common for clients to just need the isolate, so we make that easy.
+inline bool GetNextArgument(Arguments* args, int create_flags,
+                            bool is_first, v8::Isolate** result) {
+  *result = args->isolate();
+  return true;
+}
+
 
 // DispatchToCallback converts all the JavaScript arguments to C++ types and
 // invokes the base::Callback.