bind-gen: Refactor collect_forward_decls_and_include_headers

It turned out that a single function
collect_forward_decls_and_include_headers does the job for
IDL callback functions, IDL callback interfaces, IDL dictionaries,
and upcoming IDL unions, so moves the function into
codegen_utils.py to be shared among them.

Bug: 839389
Change-Id: Ieee0e75388a90367c04da9c2adb40ce4bfad072a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521873


Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824827}