Timeline: add UpdateLayerTree trace event to match InspectorInstrumentation one

We somehow missed it while adding the bulk of events for trace-based timeline.

BUG=361045

Review URL: https://codereview.chromium.org/378273002

git-svn-id: svn://svn.chromium.org/blink/trunk@177750 bbb929c8-8fbe-4397-9dbb-9b2b20218538