Initial IndexedDB Support (#33044)

Adds indexeddb support to servo. At the moment heed is being used as the
backend, although this can be swapped out by implementing `KvsEngine`.
This PR adds a thread + a thread pool for Indexeddb related operations.
Also `database_access_task_source` is added for Indexeddb related
operations.

This is a partial rewrite of #25214. (Reopened due to branching issue)

Fixes #6963

---------

Signed-off-by: Ashwin Naren <arihant2math@gmail.com>
Signed-off-by: Josh Matthews <josh@joshmatthews.net>
Co-authored-by: Rasmus Viitanen <rasviitanen@gmail.com>
Co-authored-by: Josh Matthews <josh@joshmatthews.net>

components/script_bindings/webidls/IDBFactory.webidl

@@ -0,0 +1,31 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+/*
+ * The origin of this IDL file is
+ * https://w3c.github.io/IndexedDB/#idbfactory
+ *
+ */
+
+// https://w3c.github.io/IndexedDB/#idbfactory
+partial interface mixin WindowOrWorkerGlobalScope {
+  [Pref="dom_indexeddb_enabled", SameObject] readonly attribute IDBFactory indexedDB;
+};
+
+// https://w3c.github.io/IndexedDB/#idbfactory
+[Pref="dom_indexeddb_enabled", Exposed=(Window,Worker)]
+interface IDBFactory {
+  [NewObject, Throws] IDBOpenDBRequest open(DOMString name,
+                                    optional [EnforceRange] unsigned long long version);
+  [NewObject, Throws] IDBOpenDBRequest deleteDatabase(DOMString name);
+
+  // Promise<sequence<IDBDatabaseInfo>> databases();
+
+  // short cmp(any first, any second);
+};
+
+// https://w3c.github.io/IndexedDB/#dictdef-idbdatabaseinfo
+dictionary IDBDatabaseInfo {
+  DOMString name;
+  unsigned long long version;
+};