IDLCompiler: Set correct component on Union

A user defined type is defined with a non-partial and non-mixin
definition of an IDL definition.
Following it, this CL defines a Union's component as a set of components
where its member types are defined.


Bug: 839389
Change-Id: I8632364d72cc33680ed30086f42777bb42f0ce43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2026554
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736261}

third_party/blink/renderer/bindings/scripts/web_idl/union.py

@@ -72,13 +72,13 @@ class Union(WithIdentifier, WithCodeGeneratorInfo, WithComponent,
 
         components = set()
 
-        def collect_components(idl_type):
-            user_defined_type = idl_type.type_definition_object
-            if user_defined_type:
-                components.update(user_defined_type.components)
+        def collect_primary_component(idl_type):
+            type_definition_object = idl_type.type_definition_object
+            if type_definition_object:
+                components.add(type_definition_object.components[0])
 
         for idl_type in flattened_members:
-            idl_type.apply_to_all_composing_elements(collect_components)
+            idl_type.apply_to_all_composing_elements(collect_primary_component)
 
         # TODO(peria, yukishiino): Produce unique union names.  Trying to
         # produce the names compatible to the old bindings generator for the