Add support for building "ios_chrome_unittests" with gn.

ios/chrome/app/theme/BUILD.gn

Index: ios/chrome/app/theme/BUILD.gn
diff --git a/ios/chrome/app/theme/BUILD.gn b/ios/chrome/app/theme/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..23834adca5c1ff734687fd0f1244386e013ab440
--- /dev/null
+++ b/ios/chrome/app/theme/BUILD.gn
@@ -0,0 +1,18 @@
+# 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.
+
+import("//tools/grit/grit_rule.gni")
+
+grit("theme") {
+  source = "ios_theme_resources.grd"
+  output_dir = "$root_gen_dir/ios/chrome"
+  outputs = [
+    "grit/ios_theme_resources.h",
+    "grit/ios_theme_resources_map.cc",
+    "grit/ios_theme_resources_map.h",
+    "ios_theme_resources_100_percent.pak",
+    "ios_theme_resources_200_percent.pak",
+    "ios_theme_resources_300_percent.pak",
+  ]
+}