Added a trace event to log domains we want to use for preconnect
Added a trace event to log domains we want to use for preconnect

Summary: This will be used by dancol for opening up a fd for preconnect

Reviewed By: shikong, francis-ma

Differential Revision: D2682650

fb-gh-sync-id: 9647f61f936d5feb20cbbc9eae022027cdfeb1b8

Orvid/proxygen,Orvid/proxygen,LilMeyer/proxygen,LilMeyer/proxygen,Orvid/proxygen,LilMeyer/proxygen,LilMeyer/proxygen,Orvid/proxygen