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

chrome/browser/prefs/interceptable_pref_filter.cc

Index: chrome/browser/prefs/interceptable_pref_filter.cc
diff --git a/chrome/browser/prefs/interceptable_pref_filter.cc b/chrome/browser/prefs/interceptable_pref_filter.cc
new file mode 100644
index 0000000000000000000000000000000000000000..b04c07ddf54dbc473290976b15e1544b7b9bb8fd
--- /dev/null
+++ b/chrome/browser/prefs/interceptable_pref_filter.cc
@@ -0,0 +1,39 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/browser/prefs/interceptable_pref_filter.h"
+
+#include "base/bind.h"
+
+InterceptablePrefFilter::InterceptablePrefFilter() {}
+InterceptablePrefFilter::~InterceptablePrefFilter() {}
+
+void InterceptablePrefFilter::FilterOnLoad(
+    const PostFilterOnLoadCallback& post_filter_on_load_callback,
+    scoped_ptr<base::DictionaryValue> pref_store_contents) {
+  if (filter_on_load_interceptor_.is_null()) {
+    FinalizeFilterOnLoad(post_filter_on_load_callback,
+                         pref_store_contents.Pass(),
+                         false);
+  } else {
+    // Note, in practice (in the implementation as it was in May 2014) it would
+    // be okay to pass an unretained |this| pointer below, but in order to avoid
+    // having to augment the API everywhere to explicitly enforce the ownership
+    // model as it happens to currently be: make the relationship simpler by
+    // weakly binding the FinalizeFilterOnLoadCallback below to |this|.
+    const FinalizeFilterOnLoadCallback finalize_filter_on_load(
+        base::Bind(&InterceptablePrefFilter::FinalizeFilterOnLoad,
+                   AsWeakPtr(),
+                   post_filter_on_load_callback));
+    filter_on_load_interceptor_.Run(finalize_filter_on_load,
+                                    pref_store_contents.Pass());
+    filter_on_load_interceptor_.Reset();
+  }
+}
+
+void InterceptablePrefFilter::InterceptNextFilterOnLoad(
+    const FilterOnLoadInterceptor& filter_on_load_interceptor) {
+  DCHECK(filter_on_load_interceptor_.is_null());
+  filter_on_load_interceptor_ = filter_on_load_interceptor;
+}