Add a do_not_sync pref to ExtensionPrefs.

extensions/browser/extension_prefs.h

Index: extensions/browser/extension_prefs.h
diff --git a/extensions/browser/extension_prefs.h b/extensions/browser/extension_prefs.h
index dbd734d4aa1671fc057ddee71cf059bc85d5a421..ad1712d9850b35f9302face07bde24a08df0c145 100644
--- a/extensions/browser/extension_prefs.h
+++ b/extensions/browser/extension_prefs.h
@@ -534,6 +534,9 @@ class ExtensionPrefs : public ExtensionScopedPrefs, public KeyedService {
   // found.
   base::Time GetInstallTime(const std::string& extension_id) const;
 
+  // Returns true if the extension should not be synced.
+  bool DoNotSync(const std::string& extension_id) const;
+
   // Gets/sets the last launch time of an extension.
   base::Time GetLastLaunchTime(const std::string& extension_id) const;
   void SetLastLaunchTime(const std::string& extension_id,