Make unit_tests compile in Mac GN

chrome/app_shim/BUILD.gn

Index: chrome/app_shim/BUILD.gn
diff --git a/chrome/app_shim/BUILD.gn b/chrome/app_shim/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..c4c164b080be4efa9ac3adf2e9020bd501953e3b
--- /dev/null
+++ b/chrome/app_shim/BUILD.gn
@@ -0,0 +1,17 @@
+# Copyright 2015 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.
+
+# This is the part of the shim process compiled into Chrome. It runs only
+# in the shim process, after the shim finds and loads the Chrome
+# Framework bundle.
+source_set("app_shim") {
+  sources = [
+    "chrome_main_app_mode_mac.mm",
+  ]
+
+  deps = [
+    "//chrome/common:app_mode_app_support",
+    "//chrome:strings",
+  ]
+}