Limit beforeunload returnValue propagation to event handlers.

The required extra handling of the return value from an
'onbeforeunload' event handler,

  http://www.whatwg.org/specs/web-apps/current-work/#the-event-handler-processing-algorithm
  (step 4)

should not apply to event listeners.

R=
BUG=341476

Review URL: https://codereview.chromium.org/158783002

git-svn-id: svn://svn.chromium.org/blink/trunk@166828 bbb929c8-8fbe-4397-9dbb-9b2b20218538