Clear old ARC instance before opt-in.

chrome/browser/chromeos/arc/arc_auth_service.cc

 // Copyright 2015 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/chromeos/arc/arc_auth_service.h"
 
 #include <utility>
 
 #include "ash/shelf/shelf_delegate.h"
 #include "ash/shell.h"
@@ skipping 645 matching lines @@
 
 void ArcAuthService::DisableArc() {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
   DCHECK(profile_);
   profile_->GetPrefs()->SetBoolean(prefs::kArcEnabled, false);
 }
 
 void ArcAuthService::StartUI() {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
 
+  if (!arc_bridge_service()->stopped()) {
+    // If the user attempts to re-enable ARC while the bridge is still running
+    // data must be explicitly cleared and the user should not be able to
+    // continue until after the bridge has stopped.
+    clear_required_ = true;

[hidehiko, 2016/07/14 05:11:26]

For the record of the offline chat.

Could you take a look at an alternative way to clear the data? (Having an error
UI here sounds reasonable to me).
E.g., remembering that the user explicitly disabled the ARC, until the ABS
reports that ARC is actually stopped, etc.?

+    ShowUI(UIPage::ERROR, l10n_util::GetStringUTF16(
+                              IDS_ARC_SIGN_IN_SERVICE_UNAVAILABLE_ERROR));
+    return;
+  }
+
   SetState(State::FETCHING_CODE);
 
   if (initial_opt_in_) {
     initial_opt_in_ = false;
     ShowUI(UIPage::TERMS_PROGRESS, base::string16());
   } else {
     context_->PrepareContext();
   }
 }
 
@@ skipping 75 matching lines @@
       return os << kStateFetchingCode;
     case ArcAuthService::State::ACTIVE:
       return os << kStateActive;
     default:
       NOTREACHED();
       return os;
   }
 }
 
 }  // namespace arc