Add content renderer to the GN build

mojo/public/interfaces/shell/BUILD.gn

Index: mojo/public/interfaces/shell/BUILD.gn
diff --git a/mojo/public/js/bindings/BUILD.gn b/mojo/public/interfaces/shell/BUILD.gn
similarity index 68%
copy from mojo/public/js/bindings/BUILD.gn
copy to mojo/public/interfaces/shell/BUILD.gn
index 0f305e924ec52d3d1b6923ecc012b55a1ac69eac..c11ae70a311389cec1c7e00da4de5ddc7e4de2df 100644
--- a/mojo/public/js/bindings/BUILD.gn
+++ b/mojo/public/interfaces/shell/BUILD.gn
@@ -2,9 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-source_set("bindings") {
+import("//mojo/public/tools/bindings/mojom.gni")
+
+mojom("shell") {
   sources = [
-    "constants.cc",
-    "constants.h",
+    "shell.mojom",
   ]
 }