bind-gen: Implement IDL union (1 of N)

Implements constants, trace member function, and member
variable definitions.

Bug: 839389
Change-Id: I53d3bbeace3bdd80f7bb4734d12e40dfc56bc252
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2512324


Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824906}