cros: AutomationManagerAura::SendEvent crash fix

chrome/browser/ui/aura/accessibility/automation_manager_aura.cc

 // Copyright 2014 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 "chrome/browser/ui/aura/accessibility/automation_manager_aura.h"
 
 #include <stddef.h>
 
 #include "base/memory/singleton.h"
 #include "build/build_config.h"
@@ skipping 130 matching lines @@
     return;
   }
 
   if (processing_events_) {
     pending_events_.push_back(std::make_pair(aura_obj, event_type));
     return;
   }
   processing_events_ = true;
 
   ExtensionMsg_AccessibilityEventParams params;
-  if (!current_tree_serializer_->SerializeChanges(aura_obj, &params.update)) {
+  if (!current_tree_serializer_ ||

[dmazzoni, 2017/05/17 05:13:01]

Thanks for debugging! Looks like SendEvent was being
called before current_tree_serializer_ was first initialized.

Maybe move it up to the top, no log needed:

if (!current_tree_serializer_)
  return;

[Qiang(Joe) Xu, 2017/05/17 05:36:17]

Done.

+      !current_tree_serializer_->SerializeChanges(aura_obj, &params.update)) {
     LOG(ERROR) << "Unable to serialize one accessibility event.";
     return;
   }
 
   // Make sure the focused node is serialized.
   views::AXAuraObjWrapper* focus =
       views::AXAuraObjCache::GetInstance()->GetFocus();
   if (focus)
     current_tree_serializer_->SerializeChanges(focus, &params.update);
 
@@ skipping 63 matching lines @@
     }
   }
 
   // Otherwise, fire the event directly on the Window.
   views::AXAuraObjWrapper* window_wrapper =
       views::AXAuraObjCache::GetInstance()->GetOrCreate(window);
   if (window_wrapper)
     SendEvent(nullptr, window_wrapper, action.hit_test_event_to_fire);
 #endif
 }