Task Manager: overhaul & re-enable task manager browser tests.

chrome/browser/task_manager/notification_resource_provider.cc

Index: chrome/browser/task_manager/notification_resource_provider.cc
diff --git a/chrome/browser/task_manager/notification_resource_provider.cc b/chrome/browser/task_manager/notification_resource_provider.cc
index 0b317c5e4e95bf8b8eabb262f03cba330d2e7442..7aaad2c3739a8ec658e5e8db3f36ad084783cc61 100644
--- a/chrome/browser/task_manager/notification_resource_provider.cc
+++ b/chrome/browser/task_manager/notification_resource_provider.cc
@@ -205,8 +205,11 @@ void NotificationResourceProvider::Observe(
 
 void NotificationResourceProvider::AddToTaskManager(
     BalloonHost* balloon_host) {
+  // The resource may already be tracked, if the task manager was opened
+  // while the BalloonHost was waiting to connect.
+  if (resources_.count(balloon_host))
+    return;
   NotificationResource* resource = new NotificationResource(balloon_host);
-  DCHECK(resources_.find(balloon_host) == resources_.end());
   resources_[balloon_host] = resource;
   task_manager_->AddResource(resource);
 }