IDL: Fix union type support in cross-component partial interfaces

If a partial interface in modules extends an interface in core and uses
any union types, the generated bindings/modules/v8/V8*Partial.cpp file
needs to include UnionTypesModules.h. This did not happen, since use of
union types was only tracked per main (non-partial) interface.

BUG=240176

Review URL: https://codereview.chromium.org/684653009

git-svn-id: svn://svn.chromium.org/blink/trunk@185136 bbb929c8-8fbe-4397-9dbb-9b2b20218538