Timeline: avoid exception when there's no closing Layout record

A bunch of layout tests used to flake with "Uncaught exception in
inspector front-end: Uncaught TypeError: Cannot read property 'rootNode'
of undefined". This is apparently due to the fact that the trace is
stopped while Layout is in progress, hence we do not get the closing
record and fail upon an attempt to access event.args["endData"]

Review URL: https://codereview.chromium.org/626483004

git-svn-id: svn://svn.chromium.org/blink/trunk@183136 bbb929c8-8fbe-4397-9dbb-9b2b20218538