Delay PersistentHistogramAllocator creation until it's known to be used.

chrome/browser/metrics/chrome_metrics_service_client.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 CHROME_BROWSER_METRICS_CHROME_METRICS_SERVICE_CLIENT_H_
 #define CHROME_BROWSER_METRICS_CHROME_METRICS_SERVICE_CLIENT_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 61 matching lines @@
   void CollectFinalMetricsForLog(const base::Closure& done_callback) override;
   std::unique_ptr<metrics::MetricsLogUploader> CreateUploader(
       const base::Callback<void(int)>& on_upload_complete) override;
   base::TimeDelta GetStandardUploadInterval() override;
   base::string16 GetRegistryBackupKey() override;
   void OnPluginLoadingError(const base::FilePath& plugin_path) override;
   bool IsReportingPolicyManaged() override;
   EnableMetricsDefault GetDefaultOptIn() override;
   bool IsUMACellularUploadLogicEnabled() override;
 
+  // Persistent browser metrics need to be persisted somewhere. This constant
+  // provides a known string to be used for both the allocator's internal name
+  // and for a file on disk (relative to chrome::DIR_USER_DATA) to which they
+  // can be saved.
+  static const char kBrowserMetricsName[];
+
  private:
   explicit ChromeMetricsServiceClient(
       metrics::MetricsStateManager* state_manager);
 
   // Completes the two-phase initialization of ChromeMetricsServiceClient.
   void Initialize();
 
   // Callback that continues the init task by getting a Bluetooth Adapter.
   void OnInitTaskGotHardwareClass();
 
@@ skipping 120 matching lines @@
   // Whether this client has already uploaded profiler data during this session.
   // Profiler data is uploaded at most once per session.
   bool has_uploaded_profiler_data_;
 
   base::WeakPtrFactory<ChromeMetricsServiceClient> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ChromeMetricsServiceClient);
 };
 
 #endif  // CHROME_BROWSER_METRICS_CHROME_METRICS_SERVICE_CLIENT_H_