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/webidls/Body.webidl

@@ -0,0 +1,19 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// https://fetch.spec.whatwg.org/#body
+
+[NoInterfaceObject,
+ Exposed=(Window,Worker)]
+
+interface Body {
+  readonly attribute boolean bodyUsed;
+
+  // Servo does not support Promise at this moment.
+  // [NewObject] Promise<ArrayBuffer> arrayBuffer();
+  // [NewObject] Promise<Blob> blob();
+  // [NewObject] Promise<FormData> formData();
+  // [NewObject] Promise<JSON> json();
+  // [NewObject] Promise<USVString> text();
+};

components/script/dom/webidls/Request.webidl

@@ -0,0 +1,108 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// https://fetch.spec.whatwg.org/#request-class
+
+typedef (Request or USVString) RequestInfo;
+
+[Constructor(RequestInfo input, optional RequestInit init),
+ Exposed=(Window,Worker)]
+
+interface Request {
+  readonly attribute ByteString method;
+  readonly attribute USVString url;
+  [SameObject] readonly attribute Headers headers;
+  readonly attribute RequestType type;
+  readonly attribute RequestDestination destination;
+  readonly attribute USVString referrer;
+  readonly attribute ReferrerPolicy referrerPolicy;
+  readonly attribute RequestMode mode;
+  readonly attribute RequestCredentials credentials;
+  readonly attribute RequestCache cache;
+  readonly attribute RequestRedirect redirect;
+  readonly attribute DOMString integrity;
+  [NewObject, Throws] Request clone();
+};
+
+Request implements Body;
+
+dictionary RequestInit {
+  ByteString method;
+  HeadersInit headers;
+  BodyInit? body;
+  USVString referrer;
+  ReferrerPolicy referrerPolicy;
+  RequestMode mode;
+  RequestCredentials credentials;
+  RequestCache cache;
+  RequestRedirect redirect;
+  DOMString integrity;
+  any window; // can only be set to null
+};
+
+enum RequestType {
+  "",
+  "audio",
+  "font",
+  "image",
+  "script",
+  "style",
+  "track",
+  "video"
+};
+
+enum RequestDestination {
+  "",
+  "document",
+  "embed",
+  "font",
+  "image",
+  "manifest",
+  "media",
+  "object",
+  "report",
+  "script",
+  "serviceworker",
+  "sharedworker",
+  "style",
+  "worker",
+  "xslt"
+};
+
+enum RequestMode {
+  "navigate",
+  "same-origin",
+  "no-cors",
+  "cors"
+};
+
+enum RequestCredentials {
+  "omit",
+  "same-origin",
+  "include"
+};
+
+enum RequestCache {
+  "default",
+  "no-store",
+  "reload",
+  "no-cache",
+  "force-cache",
+  "only-if-cached"
+};
+
+enum RequestRedirect {
+  "follow",
+  "error",
+  "manual"
+};
+
+enum ReferrerPolicy {
+  "",
+  "no-referrer",
+  "no-referrer-when-downgrade",
+  "origin",
+  "origin-when-cross-origin",
+  "unsafe-url"
+};