IDL Compiler: Implement ExtendedAttribute(s)

Implement ExtendedAttribute and ExtendedAttributes class
to follow the designed APIs.
https://docs.google.com/document/d/1ZPuPWjBSQVfVeJYptk7dYg1OAxx89-kwHz6oJlZGA5o/edit?usp=sharing

Bug: 839389
Change-Id: I21e2f4dd02433d52c14b362bd9f2cafd4eee3667
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1520518
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642390}
2 files changed