Extract TraceEventDispatcher from TimelineTraceEventProcessor

...move remains into Timeline agent. This removes all timeline-specific
logic from TraceEventDispatcher and allows for re-using it in other agents.

BUG=

Review URL: https://codereview.chromium.org/97123003

git-svn-id: svn://svn.chromium.org/blink/trunk@163133 bbb929c8-8fbe-4397-9dbb-9b2b20218538