Making TimeToConnect metric more robust

We have seen large percentage of WebRTC calls where the TimeToConnect goes larger than 180 sec. By looking at the code, one reason that this happens is that if kIceConnectionChecking is not fired, the current code will report a huge number.

I haven't yet found a place where Checking state will be skipped but this is just to make the code more robust.

BUG=webrtc:4278

Review URL: https://codereview.chromium.org/1155833006

Cr-Commit-Position: refs/heads/master@{#333334}
1 file changed