DevTools: don't call _updateBoundaries in tracing based Timeline

BUG=361045
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/625783003

git-svn-id: svn://svn.chromium.org/blink/trunk@183181 bbb929c8-8fbe-4397-9dbb-9b2b20218538