terminal: close split when terminal closes
terminal: close split when terminal closes

Change-Id: I127513e7a7f5beab90ad82b6800639d9ea2a85bd
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/1898352
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Joel Hockey <9ecfc52dc2ffef73afac73f3495f294aeddf1231@chromium.org>

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