Move RenderSVGResource into the inheritance hierarchy

With RenderSVGResource reduced to a common base for paint-servers,
it's possible to get rid of the multiple inheritance on
RenderSVGResourceContainer by instead making it the base of
RenderSVGResource{Gradient,Pattern} while inheriting from
RenderSVGResourceContainer.
Rename the files appropriately and update existing references to
RenderSVGResource. Also take the opportunity to tighten the types
a bit in some places.

BUG=420022

Review URL: https://codereview.chromium.org/673753004

git-svn-id: svn://svn.chromium.org/blink/trunk@184278 bbb929c8-8fbe-4397-9dbb-9b2b20218538