Move destroyLayer call to RenderLayerModelObject

There was a FIXME about it in RenderObject::willBeDestroyed
Moving to RenderLayerModelObject::willBeDestroyed keeps a very
similar timing and removes the layering violation.

While touching the code, turned RenderLayer into an OwnPtr.

Review URL: https://codereview.chromium.org/66083002

git-svn-id: svn://svn.chromium.org/blink/trunk@161699 bbb929c8-8fbe-4397-9dbb-9b2b20218538