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..487941efb8198ee5c321b8c1caeee3afcba7dc08 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_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_) {