Make UpdateLayerTree events visible on tracing based Timeline

BUG=361045
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/483743002

git-svn-id: svn://svn.chromium.org/blink/trunk@180526 bbb929c8-8fbe-4397-9dbb-9b2b20218538