DevTools: add contrastIssues experiment to enums.xml

Frontend CL: https://crrev.com/c/2662977

Bug: chromium:1155460
Change-Id: I222670febbc047199adf7c495d6929a421fd4ed0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2663176


Reviewed-by: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Alex Rudenko <alexrudenko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#849093}