Reviewed by Alexey Proskuryakov.

        Clean up DragController
        https://bugs.webkit.org/show_bug.cgi?id=25926

        I renamed m_document to m_documentUnderMouse to better document what it does
        I could call it m_lastDocumentUnderMouse to be most-accurate, but this seemed
        long enough.

        I also saw copy/paste code involving clearing the selection when moving out of
        one document and into another.  I moved that code into a function called
        mouseMovedIntoDocument.

        I also got rid of a couple needless null-checks after confirming via source
        inspection they were not ever possible.
        In Mac WebKit it's not possible to have a Page that doesn't have a mainFrame()
        I left an ASSERT(m_page->mainFrame()) in case some other port ever violates this.
        It's also not possible to return a document from documentAtPoint that is not
        in a frame (as such a document would not be rendered).

        No functional changes, thus no tests.

        * page/DragController.cpp:
        (WebCore::DragController::DragController):
        (WebCore::DragController::dragIsMove):
        (WebCore::DragController::dragExited):
        (WebCore::DragController::performDrag):
        (WebCore::DragController::mouseMovedIntoDocument):
        (WebCore::DragController::dragEnteredOrUpdated):
        (WebCore::DragController::tryDocumentDrag):
        (WebCore::DragController::operationForLoad):
        (WebCore::DragController::concludeEditDrag):
        (WebCore::DragController::canProcessDrag):
        (WebCore::DragController::tryDHTMLDrag):
        (WebCore::DragController::placeDragCaret):
        * page/DragController.h:
        (WebCore::DragController::documentUnderMouse):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@43968 268f45cc-cd09-0410-ab3c-d52691b4dbfc