Add "Keep chrome running in background" preference.

chrome/common/pref_names.cc

Index: chrome/common/pref_names.cc
diff --git a/chrome/common/pref_names.cc b/chrome/common/pref_names.cc
index 603ccf6d8170fb9ca891caff582858c3042a5732..9200f857e99205344c2253bc5a45d1c2b766f0ef 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -1350,6 +1350,9 @@ const char kManagedPopupsBlockedForUrls[] =
 // uninstalling background apps.
 const char kUserCreatedLoginItem[] = "background_mode.user_created_login_item";
 
+// Set to true if background mode is enabled on this browser.
+const char kBackgroundModeEnabled[] = "background_mode.enabled";
+
 // List of protocol handlers.
 const char kRegisteredProtocolHandlers[] =
   "custom_handlers.registered_protocol_handlers";