Expand the JsonPrefStore's interface to allow for an alternate filename to be specified.

base/prefs/json_pref_store.h

Index: base/prefs/json_pref_store.h
diff --git a/base/prefs/json_pref_store.h b/base/prefs/json_pref_store.h
index 49e74ee23a5c5273999354634c2d99b2a1c7ae8b..6ceea6885487a0c73c2910312de60eee7a25a099 100644
--- a/base/prefs/json_pref_store.h
+++ b/base/prefs/json_pref_store.h
@@ -43,12 +43,23 @@ class BASE_PREFS_EXPORT JsonPrefStore
       const base::FilePath& pref_filename,
       base::SequencedWorkerPool* worker_pool);
 
-  // |sequenced_task_runner| is must be a shutdown-blocking task runner, ideally
-  // created by GetTaskRunnerForFile() method above.
+  // Same as the constructor below with no alternate filename.
   JsonPrefStore(const base::FilePath& pref_filename,
                 base::SequencedTaskRunner* sequenced_task_runner,
                 scoped_ptr<PrefFilter> pref_filter);
 
+  // |sequenced_task_runner| must be a shutdown-blocking task runner, ideally
+  // created by the GetTaskRunnerForFile() method above.
+  // |pref_filename| is the path to the file to read prefs from.
+  // |pref_alternate_filename| is the path to an alternate file which the
+  // desired prefs may have previously been written to. If |pref_filename|
+  // doesn't exist and |pref_alternate_filename| does, |pref_alternate_filename|
+  // will be moved to |pref_filename| before the read occurs.
+  JsonPrefStore(const base::FilePath& pref_filename,
+                const base::FilePath& pref_alternate_filename,
+                base::SequencedTaskRunner* sequenced_task_runner,
+                scoped_ptr<PrefFilter> pref_filter);
+
   // PrefStore overrides:
   virtual bool GetValue(const std::string& key,
                         const base::Value** result) const OVERRIDE;
@@ -115,7 +126,8 @@ class BASE_PREFS_EXPORT JsonPrefStore
                         scoped_ptr<base::DictionaryValue> prefs,
                         bool schedule_write);
 
-  base::FilePath path_;
+  const base::FilePath path_;
+  const base::FilePath alternate_path_;
   const scoped_refptr<base::SequencedTaskRunner> sequenced_task_runner_;
 
   scoped_ptr<base::DictionaryValue> prefs_;