hterm: convert Size/RowCol to ES6 classes
hterm: convert Size/RowCol to ES6 classes

This makes the code a bit easier to manage, and will be easier to
convert to ES6 modules in the future.

Change-Id: Id14596fdc34a8a1fe68b48f08fcc0d20e5f2883d
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/2493286
Reviewed-by: Joel Hockey <9ecfc52dc2ffef73afac73f3495f294aeddf1231@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm