Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

services/preferences/public/interfaces/preferences.mojom

 // Copyright 2016 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.
 
 module prefs.mojom;
 
 import "mojo/common/values.mojom";
 import "services/preferences/public/interfaces/preferences_configuration.mojom";
 
 const string kServiceName = "preferences";
@@ skipping 81 matching lines @@
   // be received. The client asserts that it is already connected to the
   // |already_connected_types| pref stores through some other means, so the
   // Connect call will not connect to those.
   [Sync]
   Connect(PrefRegistry pref_registry,
           array<PrefStoreType> already_connected_types) =>
       (PersistentPrefStoreConnection connection,
        map<PrefStoreType, PrefStoreConnection> connections);
 };
 
+// An update to a subcomponent of a pref.
+struct SubPrefUpdate {
+  // The path to the changed value within the pref.
+  array<string> path;
+  // The new value; a null |value| indicates a delete.
+  mojo.common.mojom.Value? value;
+};
+
+union PrefUpdateValue {
+  array<SubPrefUpdate> split_updates;
+  // An atomic update to the pref. A null |atomic_update| indicates a delete.
+  mojo.common.mojom.Value? atomic_update;
+};
+
 // An update to a pref.
 struct PrefUpdate {
   // The key of the pref being updated.
   string key;
-  // The new value; a null |value| indicates a delete.
-  mojo.common.mojom.Value? value;
-  //|flags| is a bitmask of WritablePrefStore::PrefWriteFlags.
+  // The value update.
+  PrefUpdateValue value;
+  // |flags| is a bitmask of WritablePrefStore::PrefWriteFlags.
   uint32 flags;
 };
 
 // An interface providing mutation access to a PersistentPrefStore.
 interface PersistentPrefStore {
   // Sets the values for prefs.
   SetValues(array<PrefUpdate> updates);
 
   // These mirror the C++ PersistentPrefStore methods.
   CommitPendingWrite();
@@ skipping 13 matching lines @@
   // PrefRegistry::PrefRegistrationFlags and
   // PrefRegistrySyncable::PrefRegistrationFlags.
   uint32 flags;
 };
 
 interface PrefServiceControl {
   // Initializes the pref service. This must be called before the service can
   // be used.
   Init(PersistentPrefStoreConfiguration configuration);
 };