Introduce RenderingNodeProxy

This CL creates a RenderingNodeProxy class that mediates the interaction
between the RenderTree and the DOM. When we parallelize layout, some
interactions wth the DOM (typically reads) will be allowed during layout
whereas others will not. The RenderingNodeProxy will keep us honest and prevent
us from growing additional DOM dependencies.

R=esprehn@chromium.org
BUG=289678

Review URL: https://codereview.chromium.org/23451049

git-svn-id: svn://svn.chromium.org/blink/trunk@157627 bbb929c8-8fbe-4397-9dbb-9b2b20218538