Bug 1672431 - Provide a shutdown client for IOUtils r=xpcom-reviewers…

…,nika

IOUtils now provides a shutdown client for clients to register IO jobs with
that need to run before shutdown finishes. This allows IO jobs to run during
the profileBeforeChange phase.

IOUtils' event queuing has been refactored into a singleton EventQueue class,
which is responsible for running tasks and setting up shutdown blockers. It is
now guarded behind a StaticDataMutex so that is more explicitly clear what is
and is not re-entrant. (The atomic sShutdownFinished cannot be placed behind
the mutex because we need to keep track of that state even when we do not have
an EventQueue.)

Differential Revision: https://phabricator.services.mozilla.com/D103973

dom/chrome-webidl/IOUtils.webidl

@@ -4,6 +4,8 @@
  * You can obtain one at http://mozilla.org/MPL/2.0/.
  */
 
+interface nsIAsyncShutdownClient;
+
 /**
  * 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
@@ -206,6 +208,12 @@ namespace IOUtils {
   Promise<boolean> exists(DOMString path);
 };
 
+[Exposed=Window]
+partial namespace IOUtils {
+  [Throws]
+  readonly attribute any profileBeforeChange;
+};
+
 /**
  * Options to be passed to the |IOUtils.readUTF8| method.
  */