Use a union typdef for the return type of canvas.getContext

third_party/WebKit/Source/modules/modules.gypi

 {
   'includes': [
     '../core/core_generated.gypi',
     'modules_generated.gypi',
   ],
   'variables': {
     # Experimental hooks for embedder to provide extra IDL and source files.
     #
     # Note: this is not a supported API. If you rely on this, you will be broken
     # from time to time as the code generator changes in backward incompatible
@@ skipping 315 matching lines @@
     # 'partial interface' or target (right side of) 'implements'
     'modules_dependency_idl_files': [
       'audio_output_devices/HTMLMediaElementAudioOutputDevice.idl',
       'background_sync/ServiceWorkerGlobalScopeSync.idl',
       'background_sync/ServiceWorkerRegistrationSync.idl',
       'battery/NavigatorBattery.idl',
       'beacon/NavigatorBeacon.idl',
       'bluetooth/NavigatorBluetooth.idl',
       'cachestorage/WindowCacheStorage.idl',
       'cachestorage/WorkerCacheStorage.idl',
+      'canvas/HTMLCanvasElementModule.idl',
       'canvas2d/CanvasPathMethods.idl',
       'canvas2d/MouseEventHitRegion.idl',
       'canvas2d/TouchHitRegion.idl',
       'credentialmanager/NavigatorCredentials.idl',
       'crypto/WindowCrypto.idl',
       'crypto/WorkerGlobalScopeCrypto.idl',
       'device_light/WindowDeviceLight.idl',
       'device_orientation/WindowDeviceMotion.idl',
       'device_orientation/WindowDeviceOrientation.idl',
       'donottrack/NavigatorDoNotTrack.idl',
@@ skipping 496 matching lines @@
       'cachestorage/Cache.cpp',
       'cachestorage/Cache.h',
       'cachestorage/CacheStorage.cpp',
       'cachestorage/CacheStorage.h',
       'cachestorage/CacheStorageError.cpp',
       'cachestorage/CacheStorageError.h',
       'cachestorage/GlobalCacheStorage.cpp',
       'cachestorage/GlobalCacheStorage.h',
       'cachestorage/InspectorCacheStorageAgent.cpp',
       'cachestorage/InspectorCacheStorageAgent.h',
+      'canvas/HTMLCanvasElementModule.cpp',
+      'canvas/HTMLCanvasElementModule.h',
       'canvas2d/BaseRenderingContext2D.cpp',
       'canvas2d/BaseRenderingContext2D.h',
       'canvas2d/CanvasGradient.cpp',
       'canvas2d/CanvasGradient.h',
       'canvas2d/CanvasPathMethods.cpp',
       'canvas2d/CanvasPathMethods.h',
       'canvas2d/CanvasPattern.cpp',
       'canvas2d/CanvasPattern.h',
       'canvas2d/CanvasRenderingContext2D.cpp',
       'canvas2d/CanvasRenderingContext2D.h',
@@ skipping 1096 matching lines @@
       'webaudio/AudioBasicProcessorHandlerTest.cpp',
       'webaudio/ConvolverNodeTest.cpp',
       'webaudio/DynamicsCompressorNodeTest.cpp',
       'webaudio/ScriptProcessorNodeTest.cpp',
       'webaudio/StereoPannerNodeTest.cpp',
       'websockets/DOMWebSocketTest.cpp',
       'websockets/DocumentWebSocketChannelTest.cpp',
     ],
   },
 }