Default enable abs send time bwe for CallTest

Using the single stream bwe is really bad for the screenshare
test case in particular, but would probably help in other
cases as well so enabling it by default in CallTest setup.

BUG=
R=pbos@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/43089004

Cr-Commit-Position: refs/heads/master@{#8971}