Add GN targets for mojom generator

mojo/BUILD.gn

Index: mojo/BUILD.gn
diff --git a/net/tools/tld_cleanup/BUILD.gn b/mojo/BUILD.gn
similarity index 58%
copy from net/tools/tld_cleanup/BUILD.gn
copy to mojo/BUILD.gn
index bf89c008bb03908ceafa7d5fc853a454d0fda02c..47c390b54dcf5bce18b637402e8d89a76357a781 100644
--- a/net/tools/tld_cleanup/BUILD.gn
+++ b/mojo/BUILD.gn
@@ -2,13 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-source_set("tld_cleanup") {
-  sources = [
-    "tld_cleanup_util.cc",
-    "tld_cleanup_util.h",
-  ]
+group("mojo") {
   deps = [
-    "//base",
-    "//url",
+    "//mojo/public:bindings",
+    "//mojo/public/interfaces/bindings/tests:test_interfaces",
   ]
 }
+