Mark canPlayType as unstable.

tools/dom/dom.json

 {
   "AbstractWorker": {
     "comment": "http://www.w3.org/TR/workers/#the-abstractworker-abstract-interface",
     "members": {
       "addEventListener": {},
       "dispatchEvent": {},
       "onerror": {},
       "removeEventListener": {}
     },
     "support_level": "stable"
@@ skipping 4029 matching lines @@
       "NETWORK_EMPTY": {},
       "NETWORK_IDLE": {},
       "NETWORK_LOADING": {},
       "NETWORK_NO_SOURCE": {},
       "addTextTrack": {
         "comment": "http://www.whatwg.org/specs/web-apps/current-work/multipage/the-video-element.html#dom-media-addtexttrack",
         "support_level": "experimental"
       },
       "autoplay": {},
       "buffered": {},
-      "canPlayType": {},
+      "canPlayType": {
+        "comment": "https://dvcs.w3.org/hg/html-media/raw-file/tip/encrypted-media/encrypted-media.html#dom-htmlmediaelement",
+        "dart_action": "unstable"
+      },
       "controller": {},
       "controls": {},
       "currentSrc": {},
       "currentTime": {},
       "defaultMuted": {},
       "defaultPlaybackRate": {},
       "duration": {},
       "ended": {},
       "error": {},
       "initialTime": {
@@ skipping 7716 matching lines @@
       "importStylesheet": {},
       "removeParameter": {},
       "reset": {},
       "setParameter": {},
       "transformToDocument": {},
       "transformToFragment": {}
     },
     "support_level": "nonstandard"
   }
 }