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 b9e6986c54d8648191e565c52f478c4d283041e0..45c0c0233095418557b7d88bbd22b7f89a19accf 100644
--- a/chrome/browser/chromeos/arc/arc_auth_service.cc
+++ b/chrome/browser/chromeos/arc/arc_auth_service.cc
@@ -663,6 +663,14 @@ void ArcAuthService::DisableArc() {
 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
+    // the user should not be able to continue until the bridge has stopped.
+    ShowUI(UIPage::ERROR, l10n_util::GetStringUTF16(
+                              IDS_ARC_SIGN_IN_SERVICE_UNAVAILABLE_ERROR));
+    return;
+  }
+
   SetState(State::FETCHING_CODE);
 
   if (initial_opt_in_) {