Add a "Keepalive count" column to Task Manager.

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

Index: chrome/browser/task_manager/providers/web_contents/extension_task.cc
diff --git a/chrome/browser/task_manager/providers/web_contents/extension_task.cc b/chrome/browser/task_manager/providers/web_contents/extension_task.cc
index 0d47459cfa9aa9624c977f2bd7218aadb40b7299..4ceb7b144f483ede1015e2b5ec89192c4385daf1 100644
--- a/chrome/browser/task_manager/providers/web_contents/extension_task.cc
+++ b/chrome/browser/task_manager/providers/web_contents/extension_task.cc
@@ -46,7 +46,8 @@ ExtensionTask::ExtensionTask(content::WebContents* web_contents,
     : RendererTask(GetExtensionTitle(web_contents, extension, view_type),
                    GetDefaultIcon(),
                    web_contents,
-                   web_contents->GetRenderProcessHost()) {
+                   web_contents->GetRenderProcessHost()),
+      view_type_(view_type) {
   LoadExtensionIcon(extension);
 }
 
@@ -93,6 +94,20 @@ Task::Type ExtensionTask::GetType() const {
   return Task::EXTENSION;
 }
 
+int ExtensionTask::GetKeepaliveCount() const {
+  if (view_type_ != extensions::VIEW_TYPE_EXTENSION_BACKGROUND_PAGE)

[Wez, 2017/01/23 22:56:01]

Devlin: Strictly the keepalive count is on the extension host/process, but it
seemed more natural to display it associated with the extension's background
page, since that's the Task whose lifetime matches that of the process - WDYT?

[Devlin, 2017/01/23 23:52:36]

Overall, this - exposing a single count for an extension even though the count
may be from other views - sounds fine.  That said, there are some of these
columns in the TaskManager that are shared per-process - e.g. memory, cpu, and
network are all lumped together for all items in the same process.  I'm not sure
what the TaskManager-y way of doing that is (I assume Nick knows), but perhaps
handling it at that level, instead of by special-casing the background page, is
a better fit?  We don't quite want it one-per-process (because extensions can
share a process), but perhaps we can have a one-per-extension?

Or, perhaps it totally doesn't matter.  I'll defer to Nick's opinion.

[Wez, 2017/01/23 23:59:54]

This may be moot, given site-isolation for extensions, but yes, I thought that
having this be associated with the extension rather than the process made most
sense, since that's where the keep-alive count is.

That said, presumably there is some underlying keep-alive count at the renderer
process level; perhaps that should also be exposed, on a per-TaskGroup (aka
per-process) basis?

+    return -1;
+
+  const extensions::Extension* extension =
+      extensions::ProcessManager::Get(web_contents()->GetBrowserContext())
+          ->GetExtensionForWebContents(web_contents());
+  if (!extension)

[Devlin, 2017/01/23 23:52:36]

Nick: When does this happen?  (It seems like it's copied from other places in
the file)

+    return -1;
+
+  return extensions::ProcessManager::Get(web_contents()->GetBrowserContext())
+      ->GetLazyKeepaliveCount(extension);
+}
+
 void ExtensionTask::OnExtensionIconImageChanged(extensions::IconImage* image) {
   DCHECK_EQ(extension_icon_.get(), image);