[Tracing] Stop scripting_control.html from using -webkit-user-modify
[Tracing] Stop scripting_control.html from using -webkit-user-modify

This patch removes the usage of the undesired CSS property
-webkit-user-modify from scripting_control.html, making the deprecation
of the CSS property easier.

BUG=chromium:688908

Review-Url: https://codereview.chromium.org/2692363003

catapult-project/catapult,sahiljain/catapult,catapult-project/catapult,benschmaus/catapult,sahiljain/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,catapult-project/catapult-csm,benschmaus/catapult,catapult-project/catapult,sahiljain/catapult,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult,catapult-project/catapult,benschmaus/catapult,benschmaus/catapult,sahiljain/catapult,catapult-project/catapult-csm,sahiljain/catapult,catapult-project/catapult-csm,benschmaus/catapult,benschmaus/catapult,sahiljain/catapult,catapult-project/catapult-csm