Fix <rdar://problem/5652740> Crash occurs at WebCore::Widget::getView() after draggin...

Fix <rdar://problem/5652740> Crash occurs at WebCore::Widget::getView() after dragging file into window that contains web page ( econocraft.com/flood_arch.htm )

Reviewed by Adele and John.

We hit this crash if the page reloads between DragController::dragUpdated
and DragController::performDrag, meaning that m_document starts pointing to
a now viewless document.  This is picked up by an assertion in performDrag
which I have now replaced with an assignment given that the assertion is
invalid -- it is possible for m_document to be changed between dragUpdated
performDrag

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@29313 268f45cc-cd09-0410-ab3c-d52691b4dbfc