Fix UI Component race condition on start up.
Fix UI Component race condition on start up.

Approach: Re-use the existing AsynDataFetcher class to "fetch" and use
the iframe message port.  Messages are queued until the iframe's
contents have loaded and the message port is open.

Fixes #1679.

philc/vimium,justalittlenoob/vimium,justalittlenoob/vimium,PickRelated/vimium,Robinson10240/vimium,douglas-larocca/vimium,elderbas/vimium,tuchangwei/vimium,smblott-github/vimium,PrestanceDesign/vimium,bkudria/vimium,mrmr1993/vimium,elegantwww/vimium,mrmr1993/vimium,Aleroniponi/vimium,willsALMANJ/vimium,gdh1995/vimium,Aleroniponi/vimium,PickRelated/vimium,hhsue/vimium,tuchangwei/vimium,dimatter/vimium,PrestanceDesign/vimium,Robinson10240/vimium,mrmr1993/vimium,wildeyes/vimium,willsALMANJ/vimium,zhangmin510/vimium,gdh1995/vimium,PickRelated/vimium,tuchangwei/vimium,csmalin/vimium,wildeyes/vimium,douglas-larocca/vimium,philc/vimium,borgified/vimium,hhsue/vimium,Aleroniponi/vimium,hhsue/vimium,poacher2k/vimium,decaffeinate-examples/vimium,VPashkov/vimium,zhangmin510/vimium,justalittlenoob/vimium,elegantwww/vimium,elderbas/vimium,philc/vimium,decaffeinate-examples/vimium,willsALMANJ/vimium,borgified/vimium,dimatter/vimium,bkudria/vimium,smblott-github/vimium,dimatter/vimium,elderbas/vimium,PrestanceDesign/vimium,zhangmin510/vimium,cyrixhero/vimium,wildeyes/vimium,csmalin/vimium,borgified/vimium,cyrixhero/vimium,VPashkov/vimium,Lw-Cui/vimium,gdh1995/vimium,elegantwww/vimium,Lw-Cui/vimium,poacher2k/vimium,decaffeinate-examples/vimium,smblott-github/vimium,VPashkov/vimium,csmalin/vimium,bkudria/vimium,douglas-larocca/vimium,cyrixhero/vimium,Robinson10240/vimium,Lw-Cui/vimium,poacher2k/vimium