hterm: tests: convert MockRowProvider to ES6 class
hterm: tests: convert MockRowProvider to ES6 class

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

Change-Id: Ief0d6ed85c074a44fc5a3278fac91b61ad17abc5
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/2494980
Reviewed-by: Joel Hockey <9ecfc52dc2ffef73afac73f3495f294aeddf1231@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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