Add more BUILD.gn files to build these targets with GN.

ui/metro_viewer/BUILD.gn

Index: ui/metro_viewer/BUILD.gn
diff --git a/ui/metro_viewer/BUILD.gn b/ui/metro_viewer/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..c2a623d75b2be75fbc28f14115d5af1994dbd8fc
--- /dev/null
+++ b/ui/metro_viewer/BUILD.gn
@@ -0,0 +1,17 @@
+# Copyright 2014 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("metro_viewer") {
+  sources = [
+    "ime_types.cc",
+    "ime_types.h",
+    "metro_viewer_message_generator.cc",
+    "metro_viewer_message_generator.h",
+    "metro_viewer_messages.h",
+  ]
+
+  deps = [
+    "//base",
+  ]
+}