Timeline: add trace event to style/category mapping to TracingTimelineUIUtils

We used to rely on timeline record to timeline category mapping, which is not
entirely correct, since the set of trace events is slightly different. Let's fork
the style map for the time being.

BUG=361045

Review URL: https://codereview.chromium.org/316553005

git-svn-id: svn://svn.chromium.org/blink/trunk@175481 bbb929c8-8fbe-4397-9dbb-9b2b20218538