Mode has an underlying type of int8_t. When printing it we can end up
with unprintable ascii values. This shows up with --trace-turbo, and the
json file ends up with invalid bytes, and Turbolizer cannot preview.

Change-Id: I6bc905416b6e6da35a259734233b366b858452e4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2031895Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66180}