Remove MetricsServiceObserver in favor of MetricsProvider.

components/metrics/metrics_service.cc

Index: components/metrics/metrics_service.cc
diff --git a/components/metrics/metrics_service.cc b/components/metrics/metrics_service.cc
index ca9fef4fea954d01245ca6370557befef2464fa7..91600c84c1db3de289a1b9771296826de4fe204c 100644
--- a/components/metrics/metrics_service.cc
+++ b/components/metrics/metrics_service.cc
@@ -666,20 +666,8 @@ void MetricsService::GetUptimes(PrefService* pref,
   }
 }
 
-void MetricsService::AddObserver(MetricsServiceObserver* observer) {
-  DCHECK(thread_checker_.CalledOnValidThread());
-  observers_.AddObserver(observer);
-}
-
-void MetricsService::RemoveObserver(MetricsServiceObserver* observer) {
-  DCHECK(thread_checker_.CalledOnValidThread());
-  observers_.RemoveObserver(observer);
-}
-
 void MetricsService::NotifyOnDidCreateMetricsLog() {
-  DCHECK(thread_checker_.CalledOnValidThread());
-  FOR_EACH_OBSERVER(
-      MetricsServiceObserver, observers_, OnDidCreateMetricsLog());
+  DCHECK(IsSingleThreaded());

[Ilya Sherman, 2014/09/03 00:28:40]

nit: Hmm, is this change needed for this CL?  I've forgotten why we have both
IsSingleThreaded() and the thread_checker_ -- can you remind me?

[Alexei Svitkine (slow), 2014/09/03 13:24:41]

These were the only things using |thread_checker_|, so with this change only
this method was left, so I just removed it entirely.

I don't think both are necessary and I think we can probably move
IsSingleThreaded to use a ThreadChecker, but if we do that, we should change all
the code at once - which should be a separate CL.

   for (size_t i = 0; i < metrics_providers_.size(); ++i)
     metrics_providers_[i]->OnDidCreateMetricsLog();
 }