Add property to CssStyleDeclaration to detect if a CSS value is available.

tools/dom/dom.json

 {
   "ANGLEInstancedArrays": {
     "members": {
       "VERTEX_ATTRIB_ARRAY_DIVISOR_ANGLE": {
         "support_level": "untriaged"
       },
       "drawArraysInstancedANGLE": {
         "support_level": "untriaged"
       },
       "drawElementsInstancedANGLE": {
@@ skipping 12150 matching lines @@
   },
   "TimeoutHandler": {
     "comment": "http://www.whatwg.org/specs/web-apps/2007-10-26/multipage/section-timers.html#timeouthandler",
     "members": {
       "handleEvent": {}
     },
     "support_level": "stable"
   },
   "Timing": {
     "members": {
+      "__getter__": {
+        "support_level": "untriaged"
+      },
       "__setter__": {
         "support_level": "untriaged"
       },
       "delay": {
         "support_level": "untriaged"
       },
       "direction": {
         "support_level": "untriaged"
       },
       "easing": {
@@ skipping 2277 matching lines @@
       "importStylesheet": {},
       "removeParameter": {},
       "reset": {},
       "setParameter": {},
       "transformToDocument": {},
       "transformToFragment": {}
     },
     "support_level": "nonstandard"
   }
 }