CSS @import statements can cause DocLoader to use
        a dead Frame pointer.
        https://bugs.webkit.org/show_bug.cgi?id=19618

        The fix is to get rid of the Frame pointer on DocLoader.

        I also took this opportunity to clean up Document::detach
        a little to make it clear why we clear the m_frame pointer
        there, and to note that in the future we should stop
        using Node::detach to mean "tear down the whole rendering
        tree and detach from the frame".

        Test: I don't know how to make a good test for this, the test
        we have is network timing dependent and does not make a good
        layout test.

        * dom/Document.cpp:
        (WebCore::Document::Document):
        (WebCore::Document::detach):
        (WebCore::Document::clearFramePointer):
        * dom/Document.h:
        * loader/DocLoader.cpp:
        (WebCore::DocLoader::frame):
        * loader/DocLoader.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@34815 268f45cc-cd09-0410-ab3c-d52691b4dbfc