Another swipe at resolving <rdar://problem/9125145> and https://bugs.webkit.org/show_bug.cgi?id=61494
        
Reviewed by Darin Adler.

Re-land http://trac.webkit.org/changeset/87566 with invalid ASSERTs removed.

No new tests. No change in behavior.

Instead of storing the DocumentLoader at construction and never changing it,
always calculate it based on the FrameLoader's current DocumentLoader:
* dom/Document.cpp:
(WebCore::Document::Document):
(WebCore::Document::suggestedMIMEType):
(WebCore::Document::lastModified):
(WebCore::Document::initSecurityContext):
(WebCore::Document::updateURLForPushOrReplaceState):
(WebCore::Document::loader):
* dom/Document.h:

Null-check or ASSERT that the DocumentLoader exists (or both) depending on the scenario:
* bindings/ScriptControllerBase.cpp:
(WebCore::ScriptController::executeIfJavaScriptURL):
* html/MediaDocument.cpp:
(WebCore::MediaDocument::replaceMediaElementTimerFired):
* html/PluginDocument.cpp:
(WebCore::PluginDocumentParser::createDocumentStructure):
* platform/mac/HTMLConverter.mm:
(fileWrapperForElement):

* WebCore.exp.in:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@87756 268f45cc-cd09-0410-ab3c-d52691b4dbfc