NOT FOR LANDING Experiment with allocating SVGImageForContainer on demand

NOT FOR LANDING Experiment with allocating SVGImageForContainer on demand

Delay creation of SVGImageForContainer until paint-time. If we want to
support fragment URLs, the map from layout object to
SVGImageForContainer inside ImageResource proved inadequate since one
SVGImage could be used in one layout object several times with
differing sizes (and fragment URLs, if we want to stick the fragment
URL inside SVGImageForContainer).

TODO: Only background images and <img> work. E.g. border-image and
svg:image are broken.

TODO: Only store the potential fragment inside SVGImageForContainer
and delay resolving to a complete URL as long as possible.

TODO: Memory management

BUG=128055