ASSERT d->m_defersLoading != defers on detik.com and drive.google.com
https://bugs.webkit.org/show_bug.cgi?id=111902

Reviewed by Alexey Proskuryakov.

Source/WebCore:

Test: http/tests/navigation/same-url-iframes-defer-crash.html

* loader/DocumentLoader.cpp:
(WebCore::DocumentLoader::setDefersLoading): If multiple DocumentLoaders are
   using loading the same main resource, ensure only one of them can call
   ResourceLoader::setDefersLoading.

LayoutTests:

* http/tests/navigation/same-url-iframes-defer-crash-expected.txt: Added.
* http/tests/navigation/same-url-iframes-defer-crash.html: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147228 268f45cc-cd09-0410-ab3c-d52691b4dbfc