Merge pull request #5250 from conorhastings/track-default-property

add default attribute for use with track element

src/renderers/dom/shared/HTMLDOMPropertyConfig.js

@@ -80,6 +80,7 @@ var HTMLDOMPropertyConfig = {
     crossOrigin: null,
     data: null, // For `<object />` acts as `src`.
     dateTime: MUST_USE_ATTRIBUTE,
+    default: HAS_BOOLEAN_VALUE,
     defer: HAS_BOOLEAN_VALUE,
     dir: null,
     disabled: MUST_USE_ATTRIBUTE | HAS_BOOLEAN_VALUE,