DevTools: extract GPU events API out of trace event API

Create separate API functions to start/stop and collect
GPU events.

BUG=298951
R=caseq@chromium.org, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/65383006

git-svn-id: svn://svn.chromium.org/blink/trunk@161806 bbb929c8-8fbe-4397-9dbb-9b2b20218538