DevTools: Refactor JS frame events processing

Separate JS frame events creation out of flame chart entities
construction.

R=yurys@chromium.org

Review URL: https://codereview.chromium.org/412443003

git-svn-id: svn://svn.chromium.org/blink/trunk@178676 bbb929c8-8fbe-4397-9dbb-9b2b20218538