auto merge of #5217 : zslayton/servo/master, r=jdm

@jdm This initial version has a few outstanding issues that I wanted to invite input on. Specifically:

1. I had some difficulty finding a home for the `StorageType` enum. Structs defined outside of the `script` module don't seem to be able to use the `#[jstraceable]` annotation and the `net` module (where `StorageTask` lives) doesn't have access to `script`. Per Simon Sapin's suggestion, I worked around this temporarily by creating a `TraceableStorageType` stand-in struct that was traceable and which could be translated into a regular `StorageType` when being sent to the `StorageTask`. Unsure of the best way to resolve this hack. Thoughts?

2. Apart from the `Storage` constructor used in `Window::SessionStorage` and the new `Window::LocalStorage`, there's also a method called `Storage::Constructor`. I'm unclear on what (if anything) will actually invoke this, so I'm not sure which variant of `StorageType` to use here. I've temporarily created an `Unknown` variant of `StorageType` as a placeholder.

3. I discovered that the web platform tests directory's localStorage tests. Many of them now pass despite the configured expectation that they fail. However, several do not pass. Is there a good way for me to add debug logging or otherwise get a sense of which assertion failed / what went wrong?

Thanks for your continued help!

components/script/dom/webidls/Window.webidl

@@ -134,3 +134,10 @@ interface WindowSessionStorage {
   readonly attribute Storage sessionStorage;
 };
 Window implements WindowSessionStorage;
+
+// https://html.spec.whatwg.org/multipage/webstorage.html#dom-localstorage
+[NoInterfaceObject]
+interface WindowLocalStorage {
+  readonly attribute Storage localStorage;
+};
+Window implements WindowLocalStorage;