Enable sync for the settings from the Extension Settings API.

chrome/browser/extensions/extension_service.h

Index: chrome/browser/extensions/extension_service.h
diff --git a/chrome/browser/extensions/extension_service.h b/chrome/browser/extensions/extension_service.h
index 276406cd500848793bfe8191d068724cb129ee31..7a0286a8e8f4c2395df567ba968fb75f1151b1f7 100644
--- a/chrome/browser/extensions/extension_service.h
+++ b/chrome/browser/extensions/extension_service.h
@@ -28,6 +28,7 @@
 #include "chrome/browser/extensions/extension_prefs.h"
 #include "chrome/browser/extensions/extension_permissions_api.h"
 #include "chrome/browser/extensions/extension_process_manager.h"
+#include "chrome/browser/extensions/extension_settings.h"
 #include "chrome/browser/extensions/extension_sync_data.h"
 #include "chrome/browser/extensions/extension_toolbar_model.h"
 #include "chrome/browser/extensions/extensions_quota_service.h"
@@ -55,7 +56,6 @@ class ExtensionInstallUI;
 class ExtensionManagementEventRouter;
 class ExtensionPreferenceEventRouter;
 class ExtensionServiceBackend;
-class ExtensionSettings;
 class ExtensionSyncData;
 class ExtensionToolbarModel;
 class ExtensionUpdater;
@@ -178,7 +178,6 @@ class ExtensionService
                    const CommandLine* command_line,
                    const FilePath& install_directory,
                    ExtensionPrefs* extension_prefs,
-                   ExtensionSettings* extension_settings,
                    bool autoupdate_enabled,
                    bool extensions_enabled);
 
@@ -666,7 +665,7 @@ class ExtensionService
   // Preferences for the owning profile (weak reference).
   ExtensionPrefs* extension_prefs_;
 
-  // Settings for the owning profile (weak reference).
+  // Settings for the owning profile (owned, but destroyed on the FILE thread).
   ExtensionSettings* extension_settings_;
 
   // The current list of installed extensions.