DevTools: do not compute event levels in TracingModel

The levels are the UI-level notion and depend on the event visibility.
Drive-by: introduce an event consturctor that does not require payload
to avoid creating dummy payloads when creating fake trace events.

R=yurys
BUG=

Review URL: https://codereview.chromium.org/498673003

git-svn-id: svn://svn.chromium.org/blink/trunk@180856 bbb929c8-8fbe-4397-9dbb-9b2b20218538