[fuchsia] Migrate chromium.cast to the fuchsia.web API.

This changes chromium.cast.Channel to use the Open() method rather
than the legacy OnOpened() method.

Bug: 964732
Change-Id: I55e899dce0b3ef813c36a09740c8e73822b6b8b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1580479
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653679}
5 files changed