Renaming probing_interval to bwe_period globally.

This is a follow up of https://codereview.webrtc.org/2888893002/.

Bug: None
Change-Id: Ia76903858c0a6f2801f14878980e18ae6d3b85e6
Reviewed-on: https://chromium-review.googlesource.com/646020
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Minyue Li (BackIn2018March) <minyue@webrtc.org>
Cr-Original-Commit-Position: refs/heads/master@{#19644}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: bf94fda1e4b3bff094906d8c581c62d221fc0a5e
6 files changed