Add faster probing to GCC startup

[Sean-Der]

To match the behavior of Chromium's implementation as reported by @kcaffrey

I did spend some time digging through the chromium source code, and 
it looks like they are doing a multiplicative increase with an 8%/sec increase
as well, so I think it must converge faster due to the probing it does at the start.

It looked like they might start their estimate at 30Kb/s instead of 10Kb/s, but that 
doesn't make a big difference. Even starting at 30Kb/s, with 1.08 as the multiplicative
 factor, the rate won't reach 1Mb for at least 45sec: 
https://www.wolframalpha.com/input/?i=30000+*+1.08%5Et+%3D+1000000
But chrome reaches it almost instantly, and the only thing I saw in their source that 
could explain that was the probing in the first few seconds. But there is also a lot of 
code in chromium so I haven't read it all yet

[HustCoderHu]

probe_controller.cc Maybe we need to implement this. It probes bandwidth periodly ( interval time smaller than 1s, may driven by transport_feedback msg, not sure coz i only read the code roughly) in quite short time.