idl parser: Update IDL parser to accept interface mixin

interface mixin is a new IDL feature, and has a new grammar definition.
This CL updates IDL parser to accept it.
https://heycam.github.io/webidl/#idl-interface-mixins

Bug: 781257
Change-Id: Ibb771007c9147cf868995c80db8afc4ca43b9a74
Reviewed-on: https://chromium-review.googlesource.com/954763
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#541753}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ec53e05949f7ff894bc21ac467d8a74a85df9bd3
5 files changed