Fixing flakiness of CreateDataChannelAfterNegotiate.
Fixing flakiness of CreateDataChannelAfterNegotiate.

Test worked by creating a dummy data channel just to trigger the
SDP generation, then creating two data channels after negotiation.

However the dummy data channel is then racing with the "real" data
channel to get negotiated, so they could be signaled in the reverse
of the expected order.

Fixed this by simply waiting for the dummy data channel to be
signaled before creating the other data channels.

BUG=webrtc:3980
R=pthatcher@webrtc.org, skvlad@webrtc.org

Review URL: https://codereview.webrtc.org/2112593002 .

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#13329}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: bf2f569b220347c5d07d3c7d1e967bcaa0a3aad0

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