GitLab

* Added 'cause' parameter to mute method to indicate if muted state was changed by a user or by an extension.
* Added support for audible and muted event listeners.

A future patch that relies on these changes will update an API to make the audible and muted states available to extensions.

BUG=438903

Review URL: https://codereview.chromium.org/757033005

Cr-Commit-Position: refs/heads/master@{#319574}