Add mask layer and offsetFromLayoutObject info to GraphicsLayer JSON

third_party/WebKit/LayoutTests/compositing/masks/mask-layer-size-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true
     },
     {
       "name": "LayoutBlockFlow DIV id='masked'",
       "position": [10, 10],
       "bounds": [400, 200],
       "contentsOpaque": true,
       "drawsContent": true,
-      "backgroundColor": "#000000"
+      "backgroundColor": "#000000",
+      "maskLayer": [
+        {
+          "name": "Mask Layer",
+          "bounds": [400, 200],
+          "drawsContent": true
+        }
+      ]
     }
   ]
 }