Enable DumpAccessibilityTree tests to use cross-process iframes.

content/test/accessibility_browser_test_utils.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/test/accessibility_browser_test_utils.h"
 
 #include "base/bind.h"
 #include "base/logging.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
@@ skipping 65 matching lines @@
 
 void AccessibilityNotificationWaiter::WaitForNotification() {
   loop_runner_->Run();
 
   // Each loop runner can only be called once. Create a new one in case
   // the caller wants to call this again to wait for the next notification.
   loop_runner_ = new MessageLoopRunner();
 }
 
 const ui::AXTree& AccessibilityNotificationWaiter::GetAXTree() const {
-  return *frame_host_->GetAXTreeForTesting();
+  CR_DEFINE_STATIC_LOCAL(ui::AXTree, empty_tree, ());
+  const ui::AXTree* tree = frame_host_->GetAXTreeForTesting();
+  if (tree)
+    return *tree;
+  return empty_tree;
 }
 
 void AccessibilityNotificationWaiter::OnAccessibilityEvent(
     ui::AXEvent event_type, int event_target_id) {
    if (!IsAboutBlank() && (event_to_wait_for_ == ui::AX_EVENT_NONE ||
                           event_to_wait_for_ == event_type)) {
     event_target_id_ = event_target_id;
     loop_runner_->Quit();
   }
 }
 
 bool AccessibilityNotificationWaiter::IsAboutBlank() {
   // Skip any accessibility notifications related to "about:blank",
   // to avoid a possible race condition between the test beginning
   // listening for accessibility events and "about:blank" loading.
   return GetAXTree().data().url == url::kAboutBlankURL;
 }
 
 }  // namespace content