Reviewed by Darin Adler.

Make the Document be intelligent about returning its DocumentLoader, including the possibility that
the DocumentLoader will be null.

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@87566 268f45cc-cd09-0410-ab3c-d52691b4dbfc