Fix versioned struct decode in old Mojo JS

This ensures that JS doesn't attempt to decode optional MinVersion'd
fields in structs received with a lower version number.

Fixed: 1063092,b:151950986
Change-Id: Id7e4d2c5ce2d355a6e87a38d729025929c42008f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2110670
Auto-Submit: Ken Rockot <rockot@google.com>
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751866}

mojo/public/tools/bindings/generators/js_templates/struct_definition.tmpl

@@ -140,7 +140,17 @@
 {%-     endfor %}
 {%-   else %}
 {%-     for packed_field in byte.packed_fields %}
-    val.{{packed_field.field.name}} = decoder.{{packed_field.field.kind|decode_snippet}};
+{%-       if packed_field.min_version > 0 %}
+    if (version >= {{packed_field.min_version}}) {
+      val.{{packed_field.field.name}} =
+          decoder.{{packed_field.field.kind|decode_snippet}};
+    } else {
+      val.{{packed_field.field.name}} = null;
+    }
+{%-       else %}
+    val.{{packed_field.field.name}} =
+        decoder.{{packed_field.field.kind|decode_snippet}};
+{%-       endif %}
 {%-     endfor %}
 {%-   endif %}
 {%-   if byte.is_padding %}