Add missing header files to GN files

extensions/browser/api/metrics_private/BUILD.gn

 # Copyright 2016 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.
 
 source_set("metrics_private") {
   sources = [
     "metrics_private_api.cc",
     "metrics_private_api.h",
+    "metrics_private_delegate.h",
   ]
 
   configs += [
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
   ]
 
   deps = [
     "//extensions/common/api",
   ]
 }