getViewportTopLeft: support zoomed static documentElement
getViewportTopLeft: support zoomed static documentElement

if `document.documentElement` is zoomed, Vimium's hints `<div>` are also zoomed,
and then `scrollY` may be not equal with viewport client rect's `top`.

Example:
* make the tab zoom level is 1, `documentElement`'s `zoom` style is 2
* open a page, scroll to the top left corner
* press `f`, and all things are right
* exit LinkHints mode, scroll down for 100px, and then press `f`
* this time all hints has moved down 200px from the correct place

mrmr1993/vimium,smblott-github/vimium,poacher2k/vimium,mrmr1993/vimium,decaffeinate-examples/vimium,philc/vimium,smblott-github/vimium,mrmr1993/vimium,gdh1995/vimium,poacher2k/vimium,gdh1995/vimium,poacher2k/vimium,decaffeinate-examples/vimium,philc/vimium,decaffeinate-examples/vimium,gdh1995/vimium,smblott-github/vimium,philc/vimium