CodeGen: Fix header paths of union objects

Bug: 839389
Change-Id: I282656edadc4c309878b7cd8e3b8695e2689d87f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2027012
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736277}

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

@@ -102,7 +102,13 @@ class PathManager(object):
             assert components[0] == "core"
             assert components[1] == "modules"
             self._is_cross_components = True
-            self._api_component = components[0]
+            # Union does not have to support cross-component code generation
+            # because clients of IDL union must be on an upper or same layer to
+            # any of union members.
+            if isinstance(idl_definition, web_idl.Union):
+                self._api_component = components[1]
+            else:
+                self._api_component = components[0]
             self._impl_component = components[1]
         else:
             assert False