Auto merge of #13058 - malisas:malisa-responseAPI, r=Manishearth,jdm

Response API

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This PR adds the [dom::Response](https://fetch.spec.whatwg.org/#response-class) implementation and addresses #11896.

The relevant passing tests` expectations have been updated.

In order to allow non-UTF-8-encoded status messages, `net_traits::response::Response`'s `raw_status` field has been changed from type [`Option<RawStatus>`](https://doc.servo.org/hyper/http/struct.RawStatus.html) to type `Option<(u16, Vec<u8>)>`. As a result, a few other files which rely on the `raw_status` field were affected and updated.

TODOs:
- The `body` and `trailer` methods. Relies on implementation of `ReadableStream` and `Promise`s.
- Similarly, replace the dummy constructor `_body: Option<USVString>` argument with `body: ResponseBodyInit`.
- Currently, whenever `r's response's header list` or `r's Headers object` are mentioned, I always modify the `headers_reflector` field (of type dom::Headers, or `r's Headers object`) and not the corresponding hyper::Headers list in the net_traits::Response field. A completely accurate interpretation of the spec might consider making both of these lists the same thing via a reference. [Discussion](https://github.com/whatwg/fetch/issues/358) was [had](https://github.com/servo/servo/pull/12884).

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- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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components/script/dom/webidls/Response.webidl

@@ -0,0 +1,37 @@
+/* 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/#response-class
+
+// TODO: pass 'optional ResponseBodyInit? body = null' to constructor in place of USVString
+ [Constructor(optional USVString? body = null, optional ResponseInit init),
+  Exposed=(Window,Worker)]
+interface Response {
+  [NewObject] static Response error();
+  [NewObject, Throws] static Response redirect(USVString url, optional unsigned short status = 302);
+
+  readonly attribute ResponseType type;
+
+  readonly attribute USVString url;
+  readonly attribute boolean redirected;
+  readonly attribute unsigned short status;
+  readonly attribute boolean ok;
+  readonly attribute ByteString statusText;
+  [SameObject] readonly attribute Headers headers;
+  // readonly attribute ReadableStream? body;
+  // [SameObject] readonly attribute Promise<Headers> trailer;
+
+  [NewObject, Throws] Response clone();
+};
+Response implements Body;
+
+dictionary ResponseInit {
+  unsigned short status = 200;
+  ByteString statusText = "OK";
+  HeadersInit headers;
+};
+
+enum ResponseType { "basic", "cors", "default", "error", "opaque", "opaqueredirect" };
+
+// typedef (BodyInit or ReadableStream) ResponseBodyInit;