fix a potential bug that if a child iframe is focused on Vimium initing, framesForTab[tabId][0] will not be updated
fix a potential bug that if a child iframe is focused on Vimium initing, framesForTab[tabId][0] will not be updated

gdh1995/vimium-plus,gdh1995/vimium-plus,gdh1995/vimium-plus