Timeline: fix reset logic of TracingTimelineModel & co

The way we perform reset (i.e. clear records) was
badly wrong in a couple of places:
 - we generally should only reset the objects that we own,
so TimelineTracingView should have not tried to reset TracingModel;
 - TracingTimelineModel should reset before the record is
started,not after.

BUG=361045

Review URL: https://codereview.chromium.org/326693002

git-svn-id: svn://svn.chromium.org/blink/trunk@175810 bbb929c8-8fbe-4397-9dbb-9b2b20218538