TimelinePanel: REGRESSION. Followup fix for r172735 and r172617.

It fixes problem with background event style for all events and endTime for tracing events.

BUG=361045

Review URL: https://codereview.chromium.org/254793005

git-svn-id: svn://svn.chromium.org/blink/trunk@172753 bbb929c8-8fbe-4397-9dbb-9b2b20218538