Remove MetricsServiceObserver in favor of MetricsProvider.

chrome/browser/metrics/chrome_metrics_service_accessor.h

Index: chrome/browser/metrics/chrome_metrics_service_accessor.h
diff --git a/chrome/browser/metrics/chrome_metrics_service_accessor.h b/chrome/browser/metrics/chrome_metrics_service_accessor.h
index 37b02853bc221091c3fff69856456d1adb5749f1..d64f4c6f561b026279bdb10238c57bc454bfee61 100644
--- a/chrome/browser/metrics/chrome_metrics_service_accessor.h
+++ b/chrome/browser/metrics/chrome_metrics_service_accessor.h
@@ -12,7 +12,6 @@
 #include "base/macros.h"
 #include "chrome/browser/metrics/metrics_service_accessor.h"
 
-class ChromeBrowserMetricsServiceObserver;
 class Profile;
 
 namespace {
@@ -40,7 +39,6 @@ class ChromeInternalLogSource;
 class ChromeMetricsServiceAccessor : public MetricsServiceAccessor {
  private:
   friend bool prerender::IsOmniboxEnabled(Profile* profile);
-  friend class ::ChromeBrowserMetricsServiceObserver;
   friend class ChromeRenderMessageFilter;
   friend class ::CrashesDOMHandler;
   friend class DataReductionProxyChromeSettings;