[WK merge] Hide node() below RenderSVGModelObject, use element() instead.

This is a merge of http://trac.webkit.org/changeset/155398 by
Andreas Kling <akling@apple.com>.

(Also merging Compiler.h:WTF_DELETED_FUNCTION which is used in this patch).

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    This was almost already there, just delete node() and add an element()
    that returns a SVGElement*.

    The SVG render tree is crazy casual about compile-time types, this is
    only scratching the surface.

BUG=288228
R=pdr@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23709008

git-svn-id: svn://svn.chromium.org/blink/trunk@157623 bbb929c8-8fbe-4397-9dbb-9b2b20218538