add missing files from same resvision of browser tree

BUILD.gn files are edit with little-effort, so many of the added files are
not compiled in.

This helps with future uprev.

BUG=chromium:1048060
TEST=emerge

Change-Id: I32b77ce2e40425a0a45c6672457ac736ca248ccd
diff --git a/BUILD.gn b/BUILD.gn
index b956970..6687248 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1,6 +1,9 @@
 # Copyright 2018 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
+#
+# BUILD.gn doesn't compile all files in the directory to reduce build size.
+# Missing files can be added if needed.
 
 import("//common-mk/mojom_bindings_generator.gni")
 import("//common-mk/pkg_config.gni")