Use TRYCATCH_FOR_V8STRINGRESOURCE in AbstractEventListener::invokeEventHandler()

Use TRYCATCH_FOR_V8STRINGRESOURCE in AbstractEventListener::invokeEventHandler() instead
of toWebCoreString() so that exceptions raised by ToString() are no longer ignored.
This affects the string returned by the onbeforeunload callback.

R=haraken
BUG=

Review URL: https://chromiumcodereview.appspot.com/23600034

git-svn-id: svn://svn.chromium.org/blink/trunk@157673 bbb929c8-8fbe-4397-9dbb-9b2b20218538