Move counters update into a sepearate event

Counter updates are sent as sepearate events. This allows to simplify timeline model on the front-end side and will simplify switching timeline to the trace events.

Removed JS heap counters from regular events. JS heap size changes for selected event are not shown except for GCEvent, instead new heap size graph is added to the memory view.

BUG=361045
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/237003002

git-svn-id: svn://svn.chromium.org/blink/trunk@171555 bbb929c8-8fbe-4397-9dbb-9b2b20218538