Use getTargetSize in target-size check

[straker]

We didn't do this as part of #4117 but we should update the target-size check to use the new dom.getTargetSize function and replace all uses of nodeRect with the target size.

[straker]

Part of this ticket will need to include commonizing the handling of descendants of targets that can change the true clickable size of the target (such as an image inside a link) and also shouldn't be counted as an obscuring element to the target.

See #4125 (comment) and #4125 (comment)