Add back onBack handler (#8245)
Add back onBack handler (#8245)

Accidentally removed it, which makes HeaderHoc on mobile not display
the back button.
keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client