AEC dump setting in webrtc internals.

AEC dump = WebRTC echo canceller debug recordings.

This CL
- Adds a UI (checkbox) in chrome://webrtc-internals for enabling/disabling AEC dump.
- Hooking up the UI with the underlying functionality to enable/disable.

The UI part is originally done by jiayl@.

BUG=323928
R=fischman@chromium.org, jochen@chromium.org, joi@chromium.org, tsepez@chromium.org

Review URL: https://codereview.chromium.org/112683003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241266 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241266