[cssom-view] Add methods to INPUT/TEXTAREA to return rects for INPUT/TEXTAREA selection

As an end user, I've noticed that if I press End in a textarea on a line ending in whitespace, both Chrome and Firefox place the cursor after that whitespace. But, if I navigate towards that trailing whitespace using the right arrow key, the cursor jumps from the last non-whitespace character on the line to the first column of the next line.

To address the inconsistency, I filed this issue in the Chrome bug tracker, and was asked to create an issue here to add methods to INPUT/TEXTAREA to return rects for INPUT/TEXTAREA selection, like Range#getClientRects() and getBoundingClientRect. Please see that link for more context.