doc/godoc.js: put focus on div#page when necessary so that keyboard navigation events are sent to div#page.
doc/godoc.js: put focus on div#page when necessary
so that keyboard navigation events are sent to div#page.

        Fixes issue 4233.

R=adg
CC=golang-dev
http://codereview.appspot.com/6652048

rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,d0f/go-zh,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,webfd/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,rdp/rogerpack2005-golang,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,webfd/go-zh,rflanagan/reginaldflanagan-project1,webfd/go-zh,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,scirelli/scirelli-go,bryanxu/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,scirelli/scirelli-go,rdp/rogerpack2005-golang,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,webfd/go-zh,d0f/go-zh,d0f/go-zh,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,d0f/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,bryanxu/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,d0f/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,rdp/rogerpack2005-golang,d0f/go-zh,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,scirelli/scirelli-go,sanjosh/sanjos100-tipc,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,bryanxu/go-zh,scirelli/scirelli-go,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc