Use dictionaries for context creation attributes. Eliminate custom bindings.

Source/core/html/HTMLCanvasElement.idl

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+typedef (CanvasRenderingContext2D or WebGLRenderingContext) RenderingContext;
+
 interface HTMLCanvasElement : HTMLElement {
 
     attribute long width;
     attribute long height;
 
     [Custom, RaisesException] DOMString toDataURL([TreatUndefinedAs=NullString, Default=Undefined] optional DOMString? type);
 
-    // The custom binding is needed to handle context creation attributes.
-    [Custom] any getContext([Default=Undefined] optional DOMString contextId);
+    // Note: this differs deliberately from the specified Web IDL for this function:
+    //     RenderingContext? getContext(DOMString contextId, any... arguments);
+    // in order to eliminate the custom binding. It is functionally equivalent.
+    //
+    // The PermissiveDictionaryConversion extended attribute is needed to allow the
+    // autogenerated code to match the behavior of the custom binding. Web IDL
+    // requires throwing TypeError if the incoming argument is not an object type
+    // (and is not undefined or null). The binding must ignore this.
+    //
+    // Also note: the only reason this must return type "any" is to allow the
+    // InspectorInstrumentation wrappers to be called for canvases. If that could be
+    // handled differently the return type could be changed to "RenderingContext?"
+    // and the [CallWith=ScriptState] extended attribute removed.
+    [CallWith=ScriptState] any getContext([Default=Undefined] optional DOMString contextId, [PermissiveDictionaryConversion] optional CanvasContextCreationAttributes attributes);
 };