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

chrome/browser/prefs/interceptable_pref_filter.h

Index: chrome/browser/prefs/interceptable_pref_filter.h
diff --git a/chrome/browser/prefs/interceptable_pref_filter.h b/chrome/browser/prefs/interceptable_pref_filter.h
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+++ b/chrome/browser/prefs/interceptable_pref_filter.h
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+// 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.
+
+#ifndef CHROME_BROWSER_PREFS_INTERCEPTABLE_PREF_FILTER_H_
+#define CHROME_BROWSER_PREFS_INTERCEPTABLE_PREF_FILTER_H_
+
+#include "base/callback.h"
+#include "base/memory/weak_ptr.h"
+#include "base/prefs/pref_filter.h"
+#include "base/values.h"
+
+// A partial implementation of a PrefFilter whose FilterOnLoad call may be
+// intercepted by a FilterOnLoadInterceptor. Implementations of
+// InterceptablePrefFilter are expected to override FinalizeFilterOnLoad rather
+// than re-overriding FilterOnLoad.
+class InterceptablePrefFilter
+    : public PrefFilter,
+      public base::SupportsWeakPtr<InterceptablePrefFilter> {
+ public:
+  // A callback to be invoked by a FilterOnLoadInterceptor when its ready to
+  // hand back the |prefs| it was handed for early filtering. |prefs_altered|
+  // indicates whether the |prefs| were actually altered by the
+  // FilterOnLoadInterceptor before being handed back.
+  typedef base::Callback<void(scoped_ptr<base::DictionaryValue> prefs,
+                              bool prefs_altered)> FinalizeFilterOnLoadCallback;
+
+  // A callback to be invoked from FilterOnLoad. It takes ownership of prefs
+  // and may modify them before handing them back to this
+  // InterceptablePrefFilter via |finalize_filter_on_load|.
+  typedef base::Callback<
+      void(const FinalizeFilterOnLoadCallback& finalize_filter_on_load,
+           scoped_ptr<base::DictionaryValue> prefs)> FilterOnLoadInterceptor;
+
+  InterceptablePrefFilter();
+  virtual ~InterceptablePrefFilter();
+
+  // PrefFilter partial implementation.
+  virtual void FilterOnLoad(
+      const PostFilterOnLoadCallback& post_filter_on_load_callback,
+      scoped_ptr<base::DictionaryValue> pref_store_contents) OVERRIDE;
+
+  // Registers |filter_on_load_interceptor| to intercept the next FilterOnLoad
+  // event. At most one FilterOnLoadInterceptor should be registered per
+  // PrefFilter.
+  void InterceptNextFilterOnLoad(
+      const FilterOnLoadInterceptor& filter_on_load_interceptor);
+
+ private:
+  // Does any extra filtering required by the implementation of this
+  // InterceptablePrefFilter and hands back the |pref_store_contents| to the
+  // initial caller of FilterOnLoad.
+  virtual void FinalizeFilterOnLoad(
+      const PostFilterOnLoadCallback& post_filter_on_load_callback,
+      scoped_ptr<base::DictionaryValue> pref_store_contents,
+      bool prefs_altered) = 0;
+
+  // Callback to be invoked only once (and subsequently reset) on the next
+  // FilterOnLoad event. It will be allowed to modify the |prefs| handed to
+  // FilterOnLoad before handing them back to this PrefHashFilter.
+  FilterOnLoadInterceptor filter_on_load_interceptor_;
+};
+
+#endif  // CHROME_BROWSER_PREFS_INTERCEPTABLE_PREF_FILTER_H_