Revert "Replace the usage of MetricsObserverInterface by RTC_HISTOGRAM_*."

This reverts commit 870bca1f418a1abf445169a638a61f9a649d557f.

Reason for revert: it breaks internal tests and builds

Original change's description:
> Replace the usage of MetricsObserverInterface by RTC_HISTOGRAM_*.
>
> We now use RTC_HISTOGRAM_* macros in system_wrappers/include/metrics.h
> to report the metrics in pc/ and p2p/ that are currently been reported
> using MetricsObserverInterface.
>
> TBR=tommi@webrtc.org
>
> Bug: webrtc:9409
> Change-Id: I47c9975402293c72250203fa1ec19eb1668766f6
> Reviewed-on: https://webrtc-review.googlesource.com/83782
> Commit-Queue: Qingsi Wang <qingsi@google.com>
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Reviewed-by: Taylor (left Google) <deadbeef@webrtc.org>
> Reviewed-by: Steve Anton <steveanton@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#23914}

TBR=steveanton@webrtc.org,deadbeef@webrtc.org,hta@webrtc.org,tommi@webrtc.org

Change-Id: I1afd92d44f3b8cf3ae9aa6e6daa9a3a272e8097f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:9409
Reviewed-on: https://webrtc-review.googlesource.com/88040
Reviewed-by: Qingsi Wang <qingsi@webrtc.org>
Commit-Queue: Qingsi Wang <qingsi@google.com>
Cr-Commit-Position: refs/heads/master@{#23916}

api/umametrics.h

@@ -176,13 +176,13 @@ class MetricsObserverInterface : public rtc::RefCountInterface {
   // number after the highest counter.
   virtual void IncrementEnumCounter(PeerConnectionEnumCounterType type,
                                     int counter,
-                                    int counter_max) = 0;
+                                    int counter_max) {}
 
   // This is used to handle sparse counters like SSL cipher suites.
   // TODO(guoweis): Remove the implementation once the dependency's interface
   // definition is updated.
   virtual void IncrementSparseEnumCounter(PeerConnectionEnumCounterType type,
-                                          int counter) = 0;
+                                          int counter);
 
   virtual void AddHistogramSample(PeerConnectionMetricsName type,
                                   int value) = 0;