Adds a MediaConstraint for the AudioOption aec_dump

Alson includes
- a test verifying that the option is set
- changed the test verifying delay_agnostic_aec option is set to use non-default value

BUG=4555
TESTED=locally through AppRTCDemo on N7 and Android One
R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/46059004

Cr-Commit-Position: refs/heads/master@{#9109}
4 files changed