src/vs/editor/common/config/editorOptions.ts/EditorEnumOption/validate
class EditorEnumOption:	public validate(input: any): V {
		if (typeof input !== 'string') {
			return this.defaultValue;
		}
		if (this._allowedValues.indexOf(<T>input) === -1) {
			return this.defaultValue;
		}
		return this._convert(<any>input);
	}