hterm: simplify char size measuring
hterm: simplify char size measuring

By using the ES6 .repeat() helper, we can collapse a good amount of
code here to the initialization code path.  This should functionally
be the same.

BUG=chromium:727957

Change-Id: Id2f4476d6b65e5f6c7e6a56885883a965d185a0e
Reviewed-on: https://chromium-review.googlesource.com/519188
Reviewed-by: Brandon Gilmore <adbfbaf7e4f927b45ce8ad3e5043a632414a833b@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

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