SPDY Ping timeout based on RTT and RTO
SPDY Ping timeout based on RTT and RTO

Summary: Add a new trave event field to track the last timeout used for SPDY
Ping

Reviewed By: @​mzlee

Differential Revision: D2403676

Orvid/proxygen,LilMeyer/proxygen,hiproz/proxygen,songfj/proxygen,hiproz/proxygen,songfj/proxygen,pueril/proxygen,KublaikhanGeek/proxygen,pueril/proxygen,songfj/proxygen,LilMeyer/proxygen,LilMeyer/proxygen,hiproz/proxygen,LilMeyer/proxygen,KublaikhanGeek/proxygen,KublaikhanGeek/proxygen,hiproz/proxygen,Orvid/proxygen,pueril/proxygen,songfj/proxygen,KublaikhanGeek/proxygen,Orvid/proxygen,Orvid/proxygen,pueril/proxygen