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.

tests/wpt/mozilla/tests/mozilla/interfaces.html

@@ -147,6 +147,7 @@ test_interfaces([
   "ProgressEvent",
   "RadioNodeList",
   "Range",
+  "Request",
   "Screen",
   "Storage",
   "StorageEvent",

tests/wpt/mozilla/tests/mozilla/interfaces.worker.js

@@ -89,6 +89,7 @@ test_interfaces([
   "ProgressEvent",
   "RadioNodeList",
   "Range",
+  "Request",
   "Screen",
   "Storage",
   "StorageEvent",