Teach HTMLImageElement that it doesn't need ::attach.

Instead of using a custom ::attach to do post insertion work
when assigning the image resource to the RenderImage, we should
just fix the logic inside RenderImage to be able to handle
not being in the tree yet when the resource is first set.

Soon attach will not be virtual anymore.

I also made RenderImage final and most methods private as there
are no longer subclasses.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/732163004