Restore WebGLRenderingContextBase#canvas (#1051)

* Restore WebGLRenderingContextBase#canvas

Closes #1050.

* feedback

baselines/dom.generated.d.ts

@@ -16482,6 +16482,7 @@ declare var WebGLRenderingContext: {
 };
 
 interface WebGLRenderingContextBase {
+    readonly canvas: HTMLCanvasElement;
     readonly drawingBufferHeight: GLsizei;
     readonly drawingBufferWidth: GLsizei;
     activeTexture(texture: GLenum): void;

inputfiles/addedTypes.json

@@ -179,6 +179,20 @@
                     ]
                 }
             },
+            "WebGLRenderingContextBase": {
+                "properties": {
+                    "property": {
+                        "canvas": {
+                            "exposed": "Window",
+
+                            // https://github.com/KhronosGroup/WebGL/pull/3296 will make these redundant
+                            "name": "canvas",
+                            "type": "HTMLCanvasElement",
+                            "readonly": true
+                        }
+                    }
+                }
+            },
             "WindowEventHandlers": {
                 "events": {
                     "event": [

inputfiles/removedTypes.json

@@ -16,6 +16,7 @@
             "WebGLRenderingContextBase": {
                 "properties": {
                     "namesakes": {
+                        // https://github.com/KhronosGroup/WebGL/pull/3296 will make this redundant
                         "canvas": null
                     }
                 }