Stop using HTMLElement.offset* properties?

Created by: cvrebert

They ignore fractional CSS pixels and CSS transforms. See https://www.w3.org/Bugs/Public/show_bug.cgi?id=29265 and https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/JlAEmQpWMWA/_uclYmHXDwAJ getBoundingClientRect() seems to be the recommended replacement.