Web Inspector: Several inspector tests are flaky beacuse of beforeonload event handler.
https://bugs.webkit.org/show_bug.cgi?id=112111

Reviewed by Alexander Pavlov.

Added removeEventListener call.
Beforeunload message was updated so that it is possible to find it's source from it.

* inspector-protocol/page/javascriptDialogEvents-expected.txt:
* inspector-protocol/page/javascriptDialogEvents.html:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@145498 268f45cc-cd09-0410-ab3c-d52691b4dbfc