Remove widgetRendererMap and Frame::frameForWidget

Apparently nothing uses the map of RenderWidgets, we should get
rid of it.

Review URL: https://codereview.chromium.org/25850002

git-svn-id: svn://svn.chromium.org/blink/trunk@158865 bbb929c8-8fbe-4397-9dbb-9b2b20218538