Debug the generic session restore failure.

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 180a72e9fd9bd2498155858d33a80bdbddc8b0b6..687d4b12ef93bb349667c5ae3bb084302efe6e6b 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -1800,6 +1800,9 @@ const char kTabStripLayoutType[] = "tab_strip_layout_type";
 // If true, cloud policy for the user is loaded once the user signs in.
 const char kLoadCloudPolicyOnSignin[] = "policy.load_cloud_policy_on_signin";
 
+// Debug information for SessionBackend.
+const char kSessionRestoreFilesCycled[] = "session_restore.files_cycled";
+
 // *************** SERVICE PREFS ***************
 // These are attached to the service process.