Introduce background.scripts feature for extension manifests.

chrome/browser/extensions/extension_process_manager.cc

Index: chrome/browser/extensions/extension_process_manager.cc
diff --git a/chrome/browser/extensions/extension_process_manager.cc b/chrome/browser/extensions/extension_process_manager.cc
index 99f975387f4963971b77a7cb7b46a286a70452a3..6b91f335b1ed44b983adf40ef89405e1d7002d14 100644
--- a/chrome/browser/extensions/extension_process_manager.cc
+++ b/chrome/browser/extensions/extension_process_manager.cc
@@ -69,8 +69,9 @@ static void CreateBackgroundHostForExtensionLoad(
   // Start the process for the master page, if it exists and we're not lazy.
   if (!CommandLine::ForCurrentProcess()->HasSwitch(
           switches::kEnableLazyBackgroundPages) &&
-      extension->background_url().is_valid())
-    manager->CreateBackgroundHost(extension, extension->background_url());
+      extension->has_background_page()) {
+    manager->CreateBackgroundHost(extension, extension->GetBackgroundURL());
+  }
 }
 
 static void CreateBackgroundHostsForProfileStartup(