hterm: update charsize CSS variables when font-family changes
hterm: update charsize CSS variables when font-family changes

... otherwise, the cursor will be on wrong position when we change the
font-family.

Bug: 1050883
Change-Id: I7afbc961aa048ea4e682e639630f7fddb0f64fe0
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/2090813
Reviewed-by: Joel Hockey <9ecfc52dc2ffef73afac73f3495f294aeddf1231@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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