Defer quitting main message loop till shutdown tasks run

This takes care of MiraclePtr warnings in VizProcessTransportFactory
upon shutdown.

Change-Id: Ie2345e53f79e81bb99f5e8f79451f9e258823929
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4836863
Reviewed-by: Peter Kvitek <kvitekp@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1192519}

headless/lib/browser/headless_browser_impl.cc

@@ -214,8 +214,12 @@ void HeadlessBrowserImpl::Shutdown() {
     content::GetIOThreadTaskRunner({})->DeleteSoon(
         FROM_HERE, system_request_context_manager_.release());
   }
+  // We might have posted task during shutdown, let these run
+  // before quitting the message loop. See ~HeadlessWebContentsImpl
+  // for additional context.
   if (quit_main_message_loop_) {
-    std::move(quit_main_message_loop_).Run();
+    BrowserMainThread()->PostTask(FROM_HERE,
+                                  std::move(quit_main_message_loop_));
   }
 }