Introduce a new framework for back-and-forth tracked/protected preferences migration.

base/files/important_file_writer.cc

Index: base/files/important_file_writer.cc
diff --git a/base/files/important_file_writer.cc b/base/files/important_file_writer.cc
index 9cbbd736e5baa5b53c2577909ffad724f30e89d4..530c9d9095b5dae64e36ff970ef36713fe14ff7a 100644
--- a/base/files/important_file_writer.cc
+++ b/base/files/important_file_writer.cc
@@ -23,6 +23,7 @@
 #include "base/metrics/histogram.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/task_runner.h"
+#include "base/task_runner_util.h"
 #include "base/threading/thread.h"
 #include "base/time/time.h"
 
@@ -99,7 +100,8 @@ ImportantFileWriter::ImportantFileWriter(
           task_runner_(task_runner),
           serializer_(NULL),
           commit_interval_(TimeDelta::FromMilliseconds(
-              kDefaultCommitIntervalMs)) {
+              kDefaultCommitIntervalMs)),
+          weak_factory_(this) {
   DCHECK(CalledOnValidThread());
   DCHECK(task_runner_.get());
 }
@@ -126,11 +128,14 @@ void ImportantFileWriter::WriteNow(const std::string& data) {
   if (HasPendingWrite())
     timer_.Stop();
 
-  if (!task_runner_->PostTask(
+  if (!base::PostTaskAndReplyWithResult(
+          task_runner_,
           FROM_HERE,
           MakeCriticalClosure(
-              Bind(IgnoreResult(&ImportantFileWriter::WriteFileAtomically),
-                   path_, data)))) {
+              Bind(&ImportantFileWriter::WriteFileAtomically,
+                   path_, data)),
+          Bind(&ImportantFileWriter::ForwardSuccessfulWrite,
+               weak_factory_.GetWeakPtr()))) {
     // Posting the task to background message loop is not expected
     // to fail, but if it does, avoid losing data and just hit the disk
     // on the current thread.
@@ -164,4 +169,18 @@ void ImportantFileWriter::DoScheduledWrite() {
   serializer_ = NULL;
 }
 
+void ImportantFileWriter::RegisterOnNextSuccessfulWriteCallback(
+    const base::Closure& on_next_successful_write) {
+  DCHECK(on_next_successful_write_.is_null());
+  on_next_successful_write_ = on_next_successful_write;
+}
+
+void ImportantFileWriter::ForwardSuccessfulWrite(bool result) {
+  DCHECK(CalledOnValidThread());
+  if (result && !on_next_successful_write_.is_null()) {
+    on_next_successful_write_.Run();
+    on_next_successful_write_.Reset();
+  }
+}
+
 }  // namespace base