Clear old ARC instance before opt-in.

chrome/browser/chromeos/arc/arc_auth_service.cc

Index: chrome/browser/chromeos/arc/arc_auth_service.cc
diff --git a/chrome/browser/chromeos/arc/arc_auth_service.cc b/chrome/browser/chromeos/arc/arc_auth_service.cc
index 6ac8869e8447b5acb705e45fd1aa9b919e218f44..5d45f210ee0a4847ba0dc0478db87934143a8406 100644
--- a/chrome/browser/chromeos/arc/arc_auth_service.cc
+++ b/chrome/browser/chromeos/arc/arc_auth_service.cc
@@ -663,6 +663,16 @@ void ArcAuthService::DisableArc() {
 void ArcAuthService::StartUI() {
   DCHECK(thread_checker.Get().CalledOnValidThread());
 
+  if (arc_bridge_service()->state() != ArcBridgeService::State::STOPPED) {

[hidehiko, 2016/07/13 08:14:55]

Could you rebase and adapt to the newest code?

+    // 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;
+    ShowUI(UIPage::ERROR, l10n_util::GetStringUTF16(
+                              IDS_ARC_SIGN_IN_SERVICE_UNAVAILABLE_ERROR));
+    return;
+  }
+
   SetState(State::FETCHING_CODE);
 
   if (initial_opt_in_) {