DevTools: add support for nestable async trace events

Nestable async events spec: https://docs.google.com/document/d/1Z2uqj59UEts5IiXX78mkdU4kd6e7kE3JUKPoDK97bVs/edit

Also, see trace event format spec: https://docs.google.com/document/d/1CvAClvFfyA5R-PhYUmn5OOQtYMH4h6I0nSsKchNAySU/edit

Review URL: https://codereview.chromium.org/622763002

git-svn-id: svn://svn.chromium.org/blink/trunk@183295 bbb929c8-8fbe-4397-9dbb-9b2b20218538