Timeline: do not rely on UpdateLayer & PaintLayer trace events

... extract layer id from webcore CompositedLayerMapping instrumentation
instead.

BUG=322207

Review URL: https://codereview.chromium.org/91183002

git-svn-id: svn://svn.chromium.org/blink/trunk@162771 bbb929c8-8fbe-4397-9dbb-9b2b20218538