Previously ProbeController would overflow int when calculating
min_bitrate_to_probe_further_bps and when probing bitrate is
above 17 Mbps. The problem was introduced in
https://codereview.webrtc.org/2504023002. Fixed ProbeController to use
int64_t internally for bitrate calculations.
BUG=6332
Review-Url: https://codereview.webrtc.org/2574533002
Cr-Commit-Position: refs/heads/master@{#15642}