GitLab

As a first step of IDL union implementation, implements mostly-
empty code generation of IDL union bindings.

Bug: 839389
Change-Id: Ia15933cfa57cd8e2f6cc5e93870ba18316dbbc3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2512329Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824337}