Make mojo dart controller depend on tonic and get building without wtf

tonic/dart_converter_wtf.cc

Index: tonic/dart_converter_wtf.cc
diff --git a/base/test/malloc_wrapper.cc b/tonic/dart_converter_wtf.cc
similarity index 57%
copy from base/test/malloc_wrapper.cc
copy to tonic/dart_converter_wtf.cc
index eb280a3eeea52f49e534c7c5e3f6f7a40e80a750..6fe5ae753b0e5af6b8e09ea59e86e6b2c42e5dfc 100644
--- a/base/test/malloc_wrapper.cc
+++ b/tonic/dart_converter_wtf.cc
@@ -2,10 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "malloc_wrapper.h"
+#include "tonic/dart_converter_wtf.h"
 
-#include <stdlib.h>
-
-void* MallocWrapper(size_t size) {
-  return malloc(size);
-}
+// This file exists to ensure dart_converter_wtf.h doesn't miss any
+// dependencies.