Update from https://crrev.com/329939

base/prefs/writeable_pref_store.h

 // 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 BASE_PREFS_WRITEABLE_PREF_STORE_H_
 #define BASE_PREFS_WRITEABLE_PREF_STORE_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/prefs/pref_store.h"
 
 namespace base {
 class Value;
 }
 
 // A pref store that can be written to as well as read from.
 class BASE_PREFS_EXPORT WriteablePrefStore : public PrefStore {
  public:
+  // PrefWriteFlags can be used to change the way a pref will be written to
+  // storage.
+  enum PrefWriteFlags : uint32 {
+    // No flags are specified.
+    DEFAULT_PREF_WRITE_FLAGS = 0,
+
+    // This marks the pref as "lossy". There is no strict time guarantee on when
+    // a lossy pref will be persisted to permanent storage when it is modified.
+    LOSSY_PREF_WRITE_FLAG = 1 << 1
+  };
+
   WriteablePrefStore() {}
 
   // Sets a |value| for |key| in the store. Assumes ownership of |value|, which
-  // must be non-NULL.
-  virtual void SetValue(const std::string& key, base::Value* value) = 0;
+  // must be non-NULL. |flags| is a bitmask of PrefWriteFlags.
+  virtual void SetValue(const std::string& key,
+                        base::Value* value,
+                        uint32 flags) = 0;
 
   // Removes the value for |key|.
-  virtual void RemoveValue(const std::string& key) = 0;
+  virtual void RemoveValue(const std::string& key, uint32 flags) = 0;
 
   // Equivalent to PrefStore::GetValue but returns a mutable value.
   virtual bool GetMutableValue(const std::string& key,
                                base::Value** result) = 0;
 
   // Triggers a value changed notification. This function needs to be called
   // if one retrieves a list or dictionary with GetMutableValue and change its
   // value. SetValue takes care of notifications itself. Note that
   // ReportValueChanged will trigger notifications even if nothing has changed.
-  virtual void ReportValueChanged(const std::string& key) = 0;
+  // |flags| is a bitmask of PrefWriteFlags.
+  virtual void ReportValueChanged(const std::string& key, uint32 flags) = 0;
 
   // Same as SetValue, but doesn't generate notifications. This is used by
   // PrefService::GetMutableUserPref() in order to put empty entries
   // into the user pref store. Using SetValue is not an option since existing
-  // tests rely on the number of notifications generated.
-  virtual void SetValueSilently(const std::string& key, base::Value* value) = 0;
+  // tests rely on the number of notifications generated. |flags| is a bitmask
+  // of PrefWriteFlags.
+  virtual void SetValueSilently(const std::string& key,
+                                base::Value* value,
+                                uint32 flags) = 0;
 
  protected:
   ~WriteablePrefStore() override {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(WriteablePrefStore);
 };
 
 #endif  // BASE_PREFS_WRITEABLE_PREF_STORE_H_