Change the background of source position markers based on the events
they link to.

Bug: v8:10644
Change-Id: I108d9f5670acdaf5835905c2b44648c0eaf6dbd0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2604708
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71970}