Let a minimum time interval pass (one bucket size) after initialization before reporting rates (to avoid rates being based on too short time intervals).

BUG=chromium:570038

Review URL: https://codereview.webrtc.org/1582333008

Cr-Original-Commit-Position: refs/heads/master@{#11455}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 799379e8c29793b9f681e61dbd1075481b77359a
2 files changed