PopupWidget: Position body/anchor using 'margin-left' instead of 'left'

Actual dimensions of the popup element will no longer be bigger than
the visible dimensions, which fixes spurious horizontal scrollbars.
Instead, they will be smaller than the visible dimensions, which will
break horizontal scrolling-into-view. Well, you can't have everything.

I don't think there's a better way to solve this unless we position
the wrapper element instead, and change all the code that depends on
the wrapper not being positioned.

Follow-up to 28f6b943f129b681d3e3a0d76f54116144593f09.

Change-Id: I8ac21fd69c5c1be3ad439998096203e3f4164226