Remove DCHECK when initializing the TabUsageRecorder.

chrome/browser/metrics/tab_usage_recorder.h

 // 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.
 
 #ifndef CHROME_BROWSER_METRICS_TAB_USAGE_RECORDER_H_
 #define CHROME_BROWSER_METRICS_TAB_USAGE_RECORDER_H_
 
 #include "base/macros.h"
 #include "chrome/browser/metrics/tab_reactivation_tracker.h"
 #include "chrome/browser/ui/browser_tab_strip_tracker.h"
 #include "chrome/browser/ui/tabs/tab_strip_model_observer.h"
 
 namespace metrics {
 
 // This class is used to record metrics about tab reactivation. Specifically,
 // it records a histogram value everytime a tab is deactivated or reactivated.
 // The ratio of both can be calculated for tabs with different properties (e.g.
 // If the tab is pinned to the tab strip) to see if they are correlated with
 // higher probability of tab reactivation.
 class TabUsageRecorder : public TabReactivationTracker::Delegate,
                          public TabStripModelObserver {
  public:
   // Needs to be public for DEFINE_WEB_CONTENTS_USER_DATA_KEY.
   class WebContentsData;
 
   // Starts recording tab usage for all browsers.
-  static void Initialize();
+  static void InitializeIfNeeded();
 
   // TabReactivationTracker::Delegate:
   void OnTabDeactivated(content::WebContents* contents) override;
   void OnTabReactivated(content::WebContents* contents) override;
 
   // TabStripModelObserver:
   void TabInsertedAt(TabStripModel* tab_strip_model,
                      content::WebContents* contents,
                      int index,
                      bool foreground) override;
@@ skipping 11 matching lines @@
 
   TabReactivationTracker tab_reactivation_tracker_;
   BrowserTabStripTracker browser_tab_strip_tracker_;
 
   DISALLOW_COPY_AND_ASSIGN(TabUsageRecorder);
 };
 
 }  // namespace metrics
 
 #endif  // CHROME_BROWSER_METRICS_TAB_USAGE_RECORDER_H_