Adding supported checks for WebGL

tools/dom/scripts/systemhtml.py

 #!/usr/bin/python
 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 """This module provides shared functionality for the system to generate
 Dart:html APIs from the IDL database."""
 
 import emitter
 import os
@@ skipping 28 matching lines @@
     'Console.trace',
     'Console.warn',
     'CSSStyleDeclaration.setProperty',
     'Element.insertAdjacentElement',
     'Element.insertAdjacentHTML',
     'Element.insertAdjacentText',
     'Element.webkitMatchesSelector',
     'Element.remove',
     'ElementEvents.mouseWheel',
     'DOMException.name',
-    'HTMLCanvasElement.getContext',
     'HTMLTableElement.createTBody',
     'IDBDatabase.transaction',
     'KeyboardEvent.initKeyboardEvent',
     'MouseEvent.offsetX',
     'MouseEvent.offsetY',
     'Navigator.language',
     'Navigator.webkitGetUserMedia',
     'URL.createObjectURL',
     'URL.revokeObjectURL',
     'WheelEvent.wheelDeltaX',
@@ skipping 30 matching lines @@
   'HTMLShadowElement': "Element.isTagSupported('shadow')",
   'HTMLTrackElement': "Element.isTagSupported('track')",
   'MediaStreamEvent': "Event._isTypeSupported('MediaStreamEvent')",
   'MediaStreamTrackEvent': "Event._isTypeSupported('MediaStreamTrackEvent')",
   'NotificationCenter': "JS('bool', '!!(window.webkitNotifications)')",
   'Performance': "JS('bool', '!!(window.performance)')",
   'SpeechRecognition': "JS('bool', '!!(window.SpeechRecognition || "
       "window.webkitSpeechRecognition)')",
   'XMLHttpRequestProgressEvent':
       "Event._isTypeSupported('XMLHttpRequestProgressEvent')",
+  'WebGLRenderingContext': "JS('bool', '!!(window.WebGLRenderingContext)')",
   'WebKitCSSMatrix': "JS('bool', '!!(window.WebKitCSSMatrix)')",
   'WebKitPoint': "JS('bool', '!!(window.WebKitPoint)')",
   'WebSocket': "JS('bool', 'typeof window.WebSocket != \"undefined\"')",
   'XSLTProcessor': "JS('bool', '!!(window.XSLTProcessor)')",
 }
 
 # Classes that offer only static methods, and therefore we should suppress
 # constructor creation.
 _static_classes = set(['Url'])
 
@@ skipping 932 matching lines @@
     for library_name in libraries:
       self._libraries[library_name] = DartLibrary(
           library_name, template_loader, library_type, output_dir)
 
   def AddFile(self, basename, library_name, path):
     self._libraries[library_name].AddFile(path)
 
   def Emit(self, emitter, auxiliary_dir):
     for lib in self._libraries.values():
       lib.Emit(emitter, auxiliary_dir)