Don't report a keepalive count for persistent pages.

extensions/browser/process_manager.h

Index: extensions/browser/process_manager.h
diff --git a/extensions/browser/process_manager.h b/extensions/browser/process_manager.h
index c192e402be669762542f6218adc92b85958d7d0e..439626b353c6cb4863048eecdad7e352153f7a50 100644
--- a/extensions/browser/process_manager.h
+++ b/extensions/browser/process_manager.h
@@ -117,6 +117,7 @@ class ProcessManager : public KeyedService,
   // the count of how many outstanding "things" are keeping the page alive.
   // When this reaches 0, we will begin the process of shutting down the page.
   // "Things" include pending events, resource loads, and API calls.
+  // Returns -1 if |extension| does not have a lazy background page.
   int GetLazyKeepaliveCount(const Extension* extension);
   void IncrementLazyKeepaliveCount(const Extension* extension);
   void DecrementLazyKeepaliveCount(const Extension* extension);