Timeline: do not remove trace object snapshots from event list of their threads

We still won't draw them as events, as we check for non-zero duration, but we'd
be able to process them in the TracingTimelineModel, e.g. to associate with other
events and frames.

BUG=361045

Review URL: https://codereview.chromium.org/323873002

git-svn-id: svn://svn.chromium.org/blink/trunk@175809 bbb929c8-8fbe-4397-9dbb-9b2b20218538