Wait for a longer time (5 seconds) before establishing the first bandwidth estimate.

This reduces the risk of getting a small initial estimate when doing combined a/v BWE, and the audio stream is received earlier than the video stream.

In addition a check is added to make sure a probe can't reduce the BWE.

R=pbos@webrtc.org

Review URL: https://codereview.webrtc.org/1219303002 .

Cr-Commit-Position: refs/heads/master@{#9560}
11 files changed