about:tor update arrow position is wrong (Retina and zoom)

In the candidate builds of Tor Browser 5.5 and 6.0a1, the update arrow does not point to the Torbutton toolbar item on a 2x (Retina) display or when content zoom is used.

I am not sure when this regressed, but it happens because the about:tor page receives a spoofed value for window.devicePixelRatio (it is always 1.0).

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