Adding WebGL context events to Canvas.

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 3263 matching lines @@
       "willValidate": {}
     },
     "support_level": "stable"
   },
   "HTMLCanvasElement": {
     "comment": "http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#the-canvas-element",
     "members": {
       "HTMLCanvasElement": {},
       "getContext": {},
       "height": {},
+      "onwebglcontextlost": {
+        "comment": "http://www.khronos.org/registry/webgl/specs/latest/#5.15.2"
+      },
+      "onwebglcontextrestored": {
+        "comment": "http://www.khronos.org/registry/webgl/specs/latest/#5.15.3"
+      },
       "toDataURL": {},
       "width": {}
     },
     "support_level": "stable"
   },
   "HTMLCollection": {
     "comment": "http://www.w3.org/TR/domcore/#interface-htmlcollection",
     "members": {
       "__getter__": {},
       "item": {},
@@ skipping 8486 matching lines @@
       "importStylesheet": {},
       "removeParameter": {},
       "reset": {},
       "setParameter": {},
       "transformToDocument": {},
       "transformToFragment": {}
     },
     "support_level": "nonstandard"
   }
 }