Revert 156654 "Add support for BeforeUnloadEvent"

> Add support for BeforeUnloadEvent
> 
> Add support for BeforeUnloadEvent as per the specification:
> http://www.whatwg.org/specs/web-apps/current-work/#beforeunloadevent
> 
> BeforeUnloadEvent has a returnValue attribute. Setting returnValue to a non-empty
> string in an event handler causes the user agent should ask the user to confirm
> that they wish to unload the document. This is equivalent to returning a non-empty
> string in the EventHandler:
> http://www.whatwg.org/specs/web-apps/current-work/#onbeforeunloadeventhandler
> 
> BeforeUnloadEvent and returnValue are already supported by IE and Firefox. Previously,
> Blink was passing a base Event type to the beforeunload event handlers instead of
> a BeforeUnloadEvent.
> 
> Note that this patch also removes support for Event.returnValue. This used to be
> an IE extension but this is no longer supported by IE (nor Firefox) so the
> compatibility risk should be low. Applications use preventDefault() nowadays to
> do this and only fallback to Event.returnValue to support older versions of IE
> (IE < 9).
> 
> BUG=277851
> 
> Review URL: https://chromiumcodereview.appspot.com/22893051

TBR=ch.dumez@sisa.samsung.com

Review URL: https://codereview.chromium.org/23382014

git-svn-id: svn://svn.chromium.org/blink/trunk@156696 bbb929c8-8fbe-4397-9dbb-9b2b20218538