Implement the Request API for the Fetch API.

This commit adds new files related to implementing the [Request
API](https://fetch.spec.whatwg.org/#request-class). This commit also
changes the expected web platform tests results. It also modifies the
following files:

components/net_traits/request.rs
HeapSizeOf is implemented in net_traits/request so that dom::request can
be used as a wrapper around net_traits::request::Request.

components/script/dom/headers.rs
Several methods are added to Headers so that request can access and
modify some of the headers fields.

components/script/dom/mod.rs

@@ -368,6 +368,7 @@ pub mod processinginstruction;
 pub mod progressevent;
 pub mod radionodelist;
 pub mod range;
+pub mod request;
 pub mod screen;
 pub mod serviceworker;
 pub mod serviceworkercontainer;