Bug 1659838: Change IOUtils shutdown blocker phase from XPCOMWillShutdown to ProfileBeforeChange r=Gijs

IOUtils is meant to act as a drop-in replacement for OS.File. Previously,
IOUtils would block shutdown at the XPCOMWillShutDown phase to allow pending
I/O tasks to finish, however, OS.File blocks for the same reason during the
ProfileBeforeChange phase.

To make IOUtils directly compatible with OS.File, we now match this behaviour.

Differential Revision: https://phabricator.services.mozilla.com/D87511

dom/chrome-webidl/IOUtils.webidl

@@ -3,6 +3,17 @@
  * 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/.
  */
+
+/**
+ * IOUtils is a simple, efficient interface for performing file I/O from a
+ * privileged chrome-only context. All asynchronous I/O tasks are run on
+ * a background thread.
+ *
+ * Pending I/O tasks will block shutdown at the |profileBeforeChange| phase.
+ * During this shutdown phase, no additional I/O tasks will be accepted --
+ * method calls to this interface will reject once shutdown has entered this
+ * phase.
+ */
 [ChromeOnly, Exposed=(Window, Worker)]
 namespace IOUtils {
  /**

dom/system/IOUtils.cpp

@@ -451,7 +451,7 @@ already_AddRefed<nsIAsyncShutdownClient> IOUtils::GetShutdownBarrier() {
     MOZ_ASSERT(svc);
 
     nsCOMPtr<nsIAsyncShutdownClient> barrier;
-    nsresult rv = svc->GetXpcomWillShutdown(getter_AddRefs(barrier));
+    nsresult rv = svc->GetProfileBeforeChange(getter_AddRefs(barrier));
     NS_ENSURE_SUCCESS(rv, nullptr);
     sBarrier = barrier;
   }