ReceiveStatisticsProxy: add some unit tests, change members from int to int64_t.

webrtc/video/receive_statistics_proxy.cc

Index: webrtc/video/receive_statistics_proxy.cc
diff --git a/webrtc/video/receive_statistics_proxy.cc b/webrtc/video/receive_statistics_proxy.cc
index 6bf9a74cf9fdc8ba9273a37f136c50131e82697e..541a183aca6c76c52ff4c03f4de0442f9d62ab04 100644
--- a/webrtc/video/receive_statistics_proxy.cc
+++ b/webrtc/video/receive_statistics_proxy.cc
@@ -442,10 +442,11 @@ void ReceiveStatisticsProxy::SampleCounter::Add(int sample) {
   ++num_samples;
 }
 
-int ReceiveStatisticsProxy::SampleCounter::Avg(int min_required_samples) const {
+int ReceiveStatisticsProxy::SampleCounter::Avg(
+    int64_t min_required_samples) const {
   if (num_samples < min_required_samples || num_samples == 0)
     return -1;
-  return sum / num_samples;
+  return static_cast<int>(sum / num_samples);
 }
 
 void ReceiveStatisticsProxy::SampleCounter::Reset() {