IDL parser: Introduce async iterable

This CL intoduces a new grammar for async iterables[1]
and also introduces "async" as a new keyword.

In Blink, now we have no async iterables, and thus this CL
does not change actual behavior of Chrome.


[1] https://heycam.github.io/webidl/#idl-async-iterable

Bug: 839389
Change-Id: Iea4a28cf3c655a74e7d3acbe46f1856116c09339
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1813118
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#698379}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 0d8e06748ca0e8c8f8b3db01d8c0a7b103bec443
3 files changed