Timeline: fix weird numbers in selection details when recording Timeline

- reset tracing model before resetting timeline model when recording Timeline,
so that we don't use stale data in event handlers invoked from TimelineModel.reset()

BUG=434626

Review URL: https://codereview.chromium.org/743853003

git-svn-id: svn://svn.chromium.org/blink/trunk@185883 bbb929c8-8fbe-4397-9dbb-9b2b20218538