Fix SpokenFeedbackTest leaks with SingleProcessMash

I've got a report saying some SpokenFeedbackTest cases fail with
some leaks when SingleProcessMash is enabled.

It can be fixed by disabling AutomationManagerAura at shutdown
since the leaked objects are allocated through some asynchronous
events after shutting down (QuitBrowsers()).

BUG=896728
TEST=interactive_ui_tests --enable-features=SingleProcessMash

Change-Id: I215c3d5990cc78653c7fd44d378f7130a137a219
Reviewed-on: https://chromium-review.googlesource.com/c/1289295Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600889}

chrome/browser/chromeos/accessibility/spoken_feedback_browsertest.cc

@@ -32,6 +32,7 @@
 #include "chrome/browser/speech/tts_controller.h"
 #include "chrome/browser/speech/tts_platform.h"
 #include "chrome/browser/ui/ash/ksv/keyboard_shortcut_viewer_util.h"
+#include "chrome/browser/ui/aura/accessibility/automation_manager_aura.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/browser_commands.h"
 #include "chrome/browser/ui/browser_window.h"
@@ -78,6 +79,7 @@ class LoggedInSpokenFeedbackTest : public InProcessBrowserTest {
 
   void TearDownOnMainThread() override {
     AccessibilityManager::SetBrailleControllerForTest(nullptr);
+    AutomationManagerAura::GetInstance()->Disable();
   }
 
   void SendKeyPress(ui::KeyboardCode key) {