Add a do_not_sync pref to ExtensionPrefs.

chrome/browser/extensions/extension_util.h

Index: chrome/browser/extensions/extension_util.h
diff --git a/chrome/browser/extensions/extension_util.h b/chrome/browser/extensions/extension_util.h
index 5cf2892c6fc9425740dd32b72d8e8c11c4258fed..452206e441ff68d62429329384d2b0fd298cc115 100644
--- a/chrome/browser/extensions/extension_util.h
+++ b/chrome/browser/extensions/extension_util.h
@@ -85,6 +85,10 @@ bool IsAppLaunchable(const std::string& extension_id,
 bool IsAppLaunchableWithoutEnabling(const std::string& extension_id,
                                     content::BrowserContext* context);
 
+// Returns true if |extension| should be synced.
+bool ShouldSyncExtension(const Extension* extension,
+                         content::BrowserContext* context);
+
 // Returns true if |app| should be synced.
 bool ShouldSyncApp(const Extension* app, content::BrowserContext* context);