Cleanup usage of history namespace in components/history

components/history/core/browser/page_usage_data.cc

 // 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 "components/history/core/browser/page_usage_data.h"
 
 #include <algorithm>
 
-PageUsageData::PageUsageData(history::SegmentID id)
-    : id_(id),
-      score_(0.0) {
+namespace history {
+
+PageUsageData::PageUsageData(SegmentID id) : id_(id), score_(0.0) {
 }
 
 PageUsageData::~PageUsageData() {
 }
 
 // static
 bool PageUsageData::Predicate(const PageUsageData* lhs,
                               const PageUsageData* rhs) {
   return lhs->GetScore() > rhs->GetScore();
 }
+
+}  // namespace history