Stop using a LazyInstance in TabStatsTracker.

Using LazyInstance seems to be discouraged (crbug.com/686866) now that
Chrome supports Thread-safe static initialization.

Change-Id: I7649ad76ada5bd2292341f46fa1955d2ae51385e
BUG: 686866,720131
Reviewed-on: https://chromium-review.googlesource.com/594491Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491100}

chrome/browser/metrics/tab_stats_tracker.cc

@@ -11,22 +11,16 @@
 
 namespace metrics {
 
-namespace {
-
-base::LazyInstance<TabStatsTracker>::Leaky g_instance =
-    LAZY_INSTANCE_INITIALIZER;
-
-}  // namespace
-
 // static
 void TabStatsTracker::Initialize() {
-  DCHECK(g_instance == static_cast<TabStatsTracker*>(nullptr));
-  g_instance.Get();
+  // Calls GetInstance() to initialize the static instance.
+  GetInstance();
 }
 
 // static
 TabStatsTracker* TabStatsTracker::GetInstance() {
-  return g_instance.Pointer();
+  static TabStatsTracker* instance = new TabStatsTracker();
+  return instance;
 }
 
 TabStatsTracker::TabStatsTracker() : total_tabs_count_(0U), browser_count_(0U) {

chrome/browser/metrics/tab_stats_tracker.h

@@ -5,7 +5,6 @@
 #ifndef CHROME_BROWSER_METRICS_TAB_STATS_TRACKER_H_
 #define CHROME_BROWSER_METRICS_TAB_STATS_TRACKER_H_
 
-#include "base/lazy_instance.h"
 #include "base/macros.h"
 #include "base/sequence_checker.h"
 #include "chrome/browser/ui/browser_list_observer.h"
@@ -32,8 +31,6 @@ class TabStatsTracker : public TabStripModelObserver,
   size_t browser_count() const { return browser_count_; }
 
  protected:
-  friend struct base::LazyInstanceTraitsBase<TabStatsTracker>;
-
   TabStatsTracker();
   ~TabStatsTracker() override;