Fix returnToViewport support for FindMode
Fix returnToViewport support for FindMode

This was broken by the move to taking input in an iframe, since the
frontend was no longer getting keydown events for text changes, and so
the viewport wasn't being scrolled back to its original position until
the mode was exiting.

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