Filter out workers started by other pages when showing Timeline based on trace events

BUG=401895
R=alph@chromium.org

Review URL: https://codereview.chromium.org/466343002

git-svn-id: svn://svn.chromium.org/blink/trunk@180167 bbb929c8-8fbe-4397-9dbb-9b2b20218538