Add a "Keepalive count" column to Task Manager.

chrome/browser/task_manager/providers/web_contents/extension_task.h

Index: chrome/browser/task_manager/providers/web_contents/extension_task.h
diff --git a/chrome/browser/task_manager/providers/web_contents/extension_task.h b/chrome/browser/task_manager/providers/web_contents/extension_task.h
index 27ad605ad1a7e6604f1a45a0c7381004d771a8d0..57fd1c8f73b2dc938531ec6114a8098a626236d0 100644
--- a/chrome/browser/task_manager/providers/web_contents/extension_task.h
+++ b/chrome/browser/task_manager/providers/web_contents/extension_task.h
@@ -28,12 +28,15 @@ class ExtensionTask
                 extensions::ViewType view_type);
   ~ExtensionTask() override;
 
-  // task_manager::RendererTask:
+  // task_manager::RendererTask
   void UpdateTitle() override;
   void UpdateFavicon() override;
   void Activate() override;
   Type GetType() const override;
 
+  // task_manager::Task
+  int GetKeepaliveCount() const override;
+
   // extensions::IconImage::Observer
   void OnExtensionIconImageChanged(extensions::IconImage* image) override;
 
@@ -52,6 +55,8 @@ class ExtensionTask
   // The favicon of the extension represented by this task.
   std::unique_ptr<extensions::IconImage> extension_icon_;
 
+  const extensions::ViewType view_type_;
+
   DISALLOW_COPY_AND_ASSIGN(ExtensionTask);
 };