Update from https://crrev.com/311076

base/prefs/pref_value_map.cc

 // Copyright (c) 2011 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 "base/prefs/pref_value_map.h"
 
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/stl_util.h"
 #include "base/values.h"
@@ skipping 49 matching lines @@
     delete entry->second;
     prefs_.erase(entry);
     return true;
   }
 
   return false;
 }
 
 void PrefValueMap::Clear() {
   STLDeleteValues(&prefs_);
-  prefs_.clear();
 }
 
 void PrefValueMap::Swap(PrefValueMap* other) {
   prefs_.swap(other->prefs_);
 }
 
 PrefValueMap::iterator PrefValueMap::begin() {
   return prefs_.begin();
 }
 
@@ skipping 66 matching lines @@
       ++other_pref;
     }
   }
 
   // Add the remaining entries.
   for ( ; this_pref != prefs_.end(); ++this_pref)
       differing_keys->push_back(this_pref->first);
   for ( ; other_pref != other->prefs_.end(); ++other_pref)
       differing_keys->push_back(other_pref->first);
 }