Remove CrossThreadUnretained wrapper when passing AudioWorkletHandler

Because AudioWorkletHandler is ThreadSafeRefCounted, wrapping with
CrossThreadUnretained() when the handler is passed via CrossThreadBind()
is unnecessary. Use scoped_refptr<> instead.

Bug: 853520
Test: All existing layout/WPT tests pass.
Change-Id: I83409b89e80a9bbb60ea98649b16efb59facbbd4
Reviewed-on: https://chromium-review.googlesource.com/1140744
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575874}
6 files changed