Flag experiment: Use Realtime thread for Audio Worklet

Use realtime thread for Audio Worklet when the flag setting is enabled.
We believe this can be a useful experiment to collect feedback from
developers.

* Blink architecture discussion:
https://groups.google.com/a/chromium.org/d/msg/platform-architecture-dev/0wS55qzWMaw/B5A1ByIeAwAJ

Bug: 813825
Change-Id: Ife6a65992b09e8d389e57be968280daa971f12e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1580221
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661465}
11 files changed