CodeGen: Use core component as default component for Union

PathManager for union types fails if a Union doesn't contain user
defined types, because its |components| is empty.

This CL makes it to set core as its default component in such cases.


Bug: 839389
Change-Id: I423f4691d0435352b0e873af5d679b898afc9465
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2004358Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732325}

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

@@ -87,6 +87,14 @@ class PathManager(object):
 
         components = sorted(idl_definition.components)  # "core" < "modules"
 
+        if len(components) == 0:
+            assert isinstance(idl_definition, web_idl.Union)
+            # Unions of built-in types, e.g. DoubleOrString, do not have a
+            # component.
+            self._is_cross_components = False
+            default_component = web_idl.Component("core")
+            self._api_component = default_component
+            self._impl_component = default_component
         if len(components) == 1:
             component = components[0]
             self._is_cross_components = False