Add experimental echo canceller type constraint to blink interface.

Also copy it to Chrome.

blink-dev discussion: https://groups.google.com/a/chromium.org/d/topic/blink-dev/ZvL_BhJjXGQ/discussion

Bug: 846270
Change-Id: I75fb55a8e71a5b8ae5afb33eb3cd617f88f82f27
Reviewed-on: https://chromium-review.googlesource.com/1071427
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Henrik Grunell <grunell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561806}
5 files changed