Auto merge of #12830 - jdm:promises, r=Ms2ger

Implement promise bindings

This implements support for using Promises in WebIDL, executing promise callbacks in batches (equivalent to running all enqueued jobs from a setTimeout(0)), and attaching native callbacks to promise objects. This is the combined work of myself, @dati91, and @mmatyas based on the following prior work in Gecko:
* Codegen.py
* Promise.webidl
* Promise.cpp/Promise.h
* PromiseNativeHandler.h

This does not implement microtasks per #4283. This allows us to make progress on testing code that requires the use of Promises right now; the microtasks work is more complicated, but also largely orthogonal to implement.

Requires https://github.com/servo/mozjs/pull/89, https://github.com/servo/rust-mozjs/pull/287, and https://github.com/servo/rust-mozjs/pull/294.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #4282
- [X] There are tests for these changes

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components/script/dom/webidls/Promise.webidl

@@ -0,0 +1,16 @@
+/* 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/. */
+
+// This interface is entirely internal to Servo, and should not be accessible to
+// web pages.
+
+callback PromiseJobCallback = void();
+
+[TreatNonCallableAsNull]
+callback AnyCallback = any (any value);
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+// Need to escape "Promise" so it's treated as an identifier.
+interface _Promise {
+};

components/script/dom/webidls/PromiseNativeHandler.webidl

@@ -0,0 +1,13 @@
+/* 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/. */
+
+// This interface is entirely internal to Servo, and should not be accessible to
+// web pages.
+
+// Hack to allow us to have JS owning and properly tracing/CCing/etc a
+// PromiseNativeHandler.
+[NoInterfaceObject,
+ Exposed=(Window,Worker)]
+interface PromiseNativeHandler {
+};

components/script/dom/webidls/TestBinding.webidl

@@ -508,9 +508,24 @@ interface TestBinding {
   [Func="TestBinding::condition_satisfied"]
   const unsigned short funcControlledConstEnabled = 0;
 
+  [Throws]
+  Promise<any> returnResolvedPromise(any value);
+  [Throws]
+  Promise<any> returnRejectedPromise(any value);
+  readonly attribute Promise<boolean> promiseAttribute;
+  void acceptPromise(Promise<DOMString> string);
+  void acceptNullablePromise(Promise<DOMString>? string);
+  Promise<any> promiseNativeHandler(SimpleCallback? resolve, SimpleCallback? reject);
+  void promiseResolveNative(Promise<any> p, any value);
+  void promiseRejectNative(Promise<any> p, any value);
+  void promiseRejectWithTypeError(Promise<any> p, USVString message);
+  void resolvePromiseDelayed(Promise<any> p, DOMString value, unsigned long long ms);
+
   void panic();
 };
 
+callback SimpleCallback = void(any value);
+
 partial interface TestBinding {
   [Pref="dom.testable_crash.enabled"]
   void crashHard();