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}

rtc_base/unittest_main.cc

@@ -20,7 +20,6 @@
 #include "rtc_base/ssladapter.h"
 #include "rtc_base/sslstreamadapter.h"
 #include "system_wrappers/include/field_trial_default.h"
-#include "system_wrappers/include/metrics_default.h"
 #include "test/field_trial.h"
 #include "test/testsupport/fileutils.h"
 
@@ -82,7 +81,6 @@ int main(int argc, char* argv[]) {
   // InitFieldTrialsFromString stores the char*, so the char array must outlive
   // the application.
   webrtc::field_trial::InitFieldTrialsFromString(FLAG_force_fieldtrials);
-  webrtc::metrics::Enable();
 
 #if defined(WEBRTC_WIN)
   if (!FLAG_default_error_handlers) {