Repro crash loading certain webarchives after r87566.

Reviewed by Oliver Hunt.

Source/WebCore: 

Test: webarchive/loading/javascript-url-iframe-crash.html

* bindings/ScriptControllerBase.cpp:
(WebCore::ScriptController::executeIfJavaScriptURL): DocumentWriter::replaceDocument can
  cause the DocumentLoader to be destroyed, so protect it with a Ref here.

LayoutTests: 

* webarchive/loading/javascript-url-iframe-crash-expected.txt: Added.
* webarchive/loading/javascript-url-iframe-crash.html: Added.
* webarchive/loading/resources/javascript-url-iframe-crash.webarchive: Added.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@87959 268f45cc-cd09-0410-ab3c-d52691b4dbfc