Aliased webkit-prefixed CSS properties are missing

In Chrome, CSS properties which are aliases (alias_for in [CSSProperties.json5](https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/css/CSSProperties.json5?q=CSSProperties.json5&sq=package:chromium&dr)) of other properties don't show up when enumerating the properties of a `CSSStyleDeclaration` instances, but they're actually still there:

```
> 'webkitUserSelect' in document.body.style
true
> Object.getOwnPropertyNames(document.body.style).indexOf('webkitUserSelect')
-1
```

This makes some CSS properties appear to be Safari-specific when they're really not. I was thinking we should just fix this as a Chrome bug, but I see Safari has some like this as well - eg. `webkitFlex`.

Perhaps we should just have a hack for CSSStyleDeclaration to explicitly look for `'webkitFoo' in document.body.style` for each `Foo` in `document.body.style`?