Use worker pool for IO in JsonPrefStore.

chrome/common/important_file_writer.cc

Index: chrome/common/important_file_writer.cc
diff --git a/chrome/common/important_file_writer.cc b/chrome/common/important_file_writer.cc
index 6a76dfbb362b83776d0c8f37e0d2bc5176e039df..fe225128effaa10c0dc43cb1a443a5e44440e3bd 100644
--- a/chrome/common/important_file_writer.cc
+++ b/chrome/common/important_file_writer.cc
@@ -89,14 +89,15 @@ void WriteToDiskTask(const FilePath& path, const std::string& data) {
 }  // namespace
 
 ImportantFileWriter::ImportantFileWriter(
-    const FilePath& path, base::MessageLoopProxy* file_message_loop_proxy)
-        : path_(path),
-          file_message_loop_proxy_(file_message_loop_proxy),
-          serializer_(NULL),
-          commit_interval_(TimeDelta::FromMilliseconds(
-              kDefaultCommitIntervalMs)) {
+    const FilePath& path,
+    base::SequencedTaskRunner* blocking_task_runner)
+    : path_(path),
+      blocking_task_runner_(blocking_task_runner),
+      serializer_(NULL),
+      commit_interval_(TimeDelta::FromMilliseconds(
+          kDefaultCommitIntervalMs)) {
   DCHECK(CalledOnValidThread());
-  DCHECK(file_message_loop_proxy_.get());
+  DCHECK(blocking_task_runner_.get());
 }
 
 ImportantFileWriter::~ImportantFileWriter() {
@@ -121,7 +122,7 @@ void ImportantFileWriter::WriteNow(const std::string& data) {
   if (HasPendingWrite())
     timer_.Stop();
 
-  if (!file_message_loop_proxy_->PostTask(
+  if (!blocking_task_runner_->PostTask(
       FROM_HERE, base::Bind(&WriteToDiskTask, path_, data))) {
     // Posting the task to background message loop is not expected
     // to fail, but if it does, avoid losing data and just hit the disk