hterm: stop matching 8-bit control codes in utf8 mode
hterm: stop matching 8-bit control codes in utf8 mode

We currently search for 8-bit control codes in utf8 data streams.
This doesn't make sense as valid utf8 encoding won't match any of
the 8-bit escape sequences.  They're also not terribly useful as
every sequence has a standard 7-bit form that programs use.  To
speed up processing, and avoid breaking up utf8 codepoints, drop
the 8-bit matching when we're in utf8 mode.

BUG=chromium:747625

Change-Id: Iae53c7bdb3b281c723c7b41d1e1284111386c9c4
Reviewed-on: https://chromium-review.googlesource.com/618547
Reviewed-by: Brandon Gilmore <adbfbaf7e4f927b45ce8ad3e5043a632414a833b@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

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