Use worker pool for IO in JsonPrefStore.

chrome/common/json_pref_store.h

Index: chrome/common/json_pref_store.h
diff --git a/chrome/common/json_pref_store.h b/chrome/common/json_pref_store.h
index 9f71a0efb8e16acb1f9c71d754a9f5a902583578..8ee5881801470a787281bc2a89f0776569fd7171 100644
--- a/chrome/common/json_pref_store.h
+++ b/chrome/common/json_pref_store.h
@@ -19,7 +19,7 @@
 
 namespace base {
 class DictionaryValue;
-class MessageLoopProxy;
+class SequencedTaskRunner;
 class Value;
 }
 
@@ -29,10 +29,10 @@ class FilePath;
 class JsonPrefStore : public PersistentPrefStore,
                       public ImportantFileWriter::DataSerializer {
  public:
-  // |file_message_loop_proxy| is the MessageLoopProxy for a thread on which
-  // file I/O can be done.
+  // |blocking_task_runner| is the SequencedTaskRunner on which file
+  // I/O can be done.
   JsonPrefStore(const FilePath& pref_filename,
-                base::MessageLoopProxy* file_message_loop_proxy);
+                base::SequencedTaskRunner* blocking_task_runner);
 
   // PrefStore overrides:
   virtual ReadResult GetValue(const std::string& key,
@@ -70,7 +70,7 @@ class JsonPrefStore : public PersistentPrefStore,
   virtual bool SerializeData(std::string* output) OVERRIDE;
 
   FilePath path_;
-  scoped_refptr<base::MessageLoopProxy> file_message_loop_proxy_;
+  scoped_refptr<base::SequencedTaskRunner> blocking_task_runner_;
 
   scoped_ptr<base::DictionaryValue> prefs_;