Instead of having to specify a bitrate and how many packets to use,
the BitrateProber will now use the bitrate to calculate how many
bytes it will use to probe that bitrate instead.
For now, |kMinProbeDurationMs| is set to 15 ms which means that probing
at 1900 kbps will result in 1900/8*0.015 = 3.5 kB used. Since we can
expect packets to be smaller at the beginning of a stream (500 to 700
bytes) this will result in 7 to 5 packets sent for that bitrate, and
should work very similar to how the current initial probing works.
A minimum of 5 packets are always sent.
BUG=webrtc:6822
Review-Url: https://codereview.webrtc.org/2609113003
Cr-Commit-Position: refs/heads/master@{#15899}
fd58b61068