bind-gen: Fix HTML{Embed,Object}Element's named definer

For a historical reason, HTML{Embed,Object}Element's named
definer depends on a fallback logic of the definer and also
the named property setter.

This patch fixes the bug with a minimal change so that it's
easy to backport it.

Bug: 1137541
Change-Id: Ic3af70604a379814fae24970317868ab89f7c4c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2467744Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816597}

third_party/blink/renderer/bindings/scripts/bind_gen/interface.py

@@ -2986,6 +2986,17 @@ if (v8_property_desc.has_get() || v8_property_desc.has_set()) {
   }
   return;
 }
+"""))
+        if cg_context.interface.identifier in ("HTMLEmbedElement",
+                                               "HTMLObjectElement"):
+            body.append(
+                TextNode("""\
+// HTMLEmbedElement and HTMLObjectElement's named properties implementation
+// depend on the default fallback behavior.  So, just fallback.
+"""))
+        else:
+            body.append(
+                TextNode("""\
 // step 2.2.2.2. Invoke the named property setter with P and Desc.[[Value]].
 ${class_name}::NamedPropertySetterCallback(
     ${v8_property_name}, ${v8_property_desc}.value(), ${info});