Put SVGResourceContainer subtypes into their own property trees.

third_party/WebKit/LayoutTests/paint/invalidation/svg/mask-invalidation-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "paintInvalidations": [
         {
-          "object": "LayoutSVGRect rect id='maskRect'",
-          "rect": [100, 100, 460, 316],
-          "reason": "subtree"
-        },
-        {
-          "object": "LayoutSVGRect rect id='maskRect'",
-          "rect": [50, 50, 460, 316],
-          "reason": "subtree"
-        },
-        {
           "object": "LayoutSVGContainer g id='root'",
           "rect": [50, 50, 453, 299],
           "reason": "bounds change"
         },
         {
           "object": "LayoutSVGRect rect",
           "rect": [50, 50, 453, 299],
           "reason": "layoutObject removal"
         }
       ]
@@ skipping 12 matching lines @@
       "object": "LayoutSVGContainer g id='root'",
       "reason": "bounds change"
     },
     {
       "object": "LayoutSVGRect rect",
       "reason": "layoutObject insertion"
     }
   ]
 }