Add a "Keepalive count" column to Task Manager.

chrome/browser/task_manager/test_task_manager.cc

Index: chrome/browser/task_manager/test_task_manager.cc
diff --git a/chrome/browser/task_manager/test_task_manager.cc b/chrome/browser/task_manager/test_task_manager.cc
index 89ba2c4e3ca615fcec7c3aada60d45e9c5b4f3f2..5b90af6f500e3eb519707ee3f29d2110fd7f274b 100644
--- a/chrome/browser/task_manager/test_task_manager.cc
+++ b/chrome/browser/task_manager/test_task_manager.cc
@@ -158,6 +158,10 @@ bool TestTaskManager::GetWebCacheStats(
   return false;
 }
 
+int TestTaskManager::GetKeepaliveCount(TaskId task_id) const {
+  return -1;
+}
+
 const TaskIdList& TestTaskManager::GetTaskIdsList() const {
   return ids_;
 }