cros: Fix MachineLearningDecisionServiceProvider shutdown crash

Release MachineLearningDecisionServiceProvider before tearing
down ash to fix the crash.

Bug: 987687
Change-Id: Ice42d1c50b0534d5f8386038087b74a84bd6969c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1719447Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681421}

chrome/browser/chromeos/chrome_browser_main_chromeos.cc

@@ -426,13 +426,17 @@ class DBusServices {
     vm_applications_service_.reset();
     drive_file_stream_service_.reset();
     cryptohome_key_delegate_service_.reset();
-    machine_learning_decision_service_.reset();
     ProcessDataCollector::Shutdown();
     PowerDataCollector::Shutdown();
     PowerPolicyController::Shutdown();
     device::BluetoothAdapterFactory::Shutdown();
   }
 
+  void PreAshShutdown() {
+    // Services depending on ash should be released here.
+    machine_learning_decision_service_.reset();
+  }
+
  private:
   std::unique_ptr<CrosDBusService> proxy_resolution_service_;
   std::unique_ptr<CrosDBusService> kiosk_info_service_;
@@ -1191,6 +1195,9 @@ void ChromeBrowserMainPartsChromeos::PostMainMessageLoopRun() {
         language::prefs::kApplicationLocale));
   }
 
+  // Cleans up dbus services depending on ash.
+  dbus_services_->PreAshShutdown();
+
   // NOTE: Closes ash and destroys ash::Shell.
   ChromeBrowserMainPartsLinux::PostMainMessageLoopRun();