DevTools: [Timeline] do not show EventDispatch records with no children

The original instrumentation filters these events in InspectorTimelineAgent.cpp
In the offline discussion we decided that tracing based implementation should
report all the events and filter such empty records on frontend.

BUG=

Review URL: https://codereview.chromium.org/390853004

git-svn-id: svn://svn.chromium.org/blink/trunk@178094 bbb929c8-8fbe-4397-9dbb-9b2b20218538