Make ui gn check pass. Misc GN build improvements.

mojo/public/tools/bindings/mojom.gni

Index: mojo/public/tools/bindings/mojom.gni
diff --git a/mojo/public/tools/bindings/mojom.gni b/mojo/public/tools/bindings/mojom.gni
index 7ee71b2e5ef1d33294d2a4ef2adf6feb3a86e4b2..f497051de43967e3c1267715f216e4e344fed572 100644
--- a/mojo/public/tools/bindings/mojom.gni
+++ b/mojo/public/tools/bindings/mojom.gni
@@ -12,8 +12,10 @@
 #       List of source .mojom files to compile.
 #
 #   deps (optional)
+#
 #   visibility (optional)
-#       Normal meaning.
+#       Normal meaning. However, this must be a list (normal visibility
+#       allows a single string).
 template("mojom") {
   assert(defined(invoker.sources),
          "\"sources\" must be defined for the $target_name template.")
@@ -75,11 +77,18 @@ template("mojom") {
     "{{source_gen_dir}}/{{source_name_part}}_mojom.py",
   ]
 
-  target_visibility = ":$target_name"
+  if (defined(invoker.visibility)) {
+    # Need to save this because the the target_name is overwritten inside the
+    # action to be that of the action itself. Only define this in the case the
+    # var is used to avoid unused var error.
+    target_visibility = ":$target_name"
+  }
 
   generator_target_name = target_name + "_generator"
   action_foreach(generator_target_name) {
-    visibility = target_visibility
+    if (defined(invoker.visibility)) {
+      visibility = [ target_visibility ] + invoker.visibility  # If you get an error here, make your visibility be a list.
+    }
     script = generator_script
     inputs = generator_sources
     sources = invoker.sources