Skip accounting for small packets and suspend the prober if no
large-enough packets have been sent for some time. This especially seems
to have triggered in audio-only calls where all packets are too small,
making TimeUntilNextProbe return 0 forever, causing the module process
thread to wake up forever.
BUG=webrtc:5506
R=stefan@webrtc.org
Review URL: https://codereview.webrtc.org/1688703002 .
Cr-Commit-Position: refs/heads/master@{#11634}
