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

webrtc/video/receive_statistics_proxy.h

Index: webrtc/video/receive_statistics_proxy.h
diff --git a/webrtc/video/receive_statistics_proxy.h b/webrtc/video/receive_statistics_proxy.h
index 33b13f7126e1fa994c82772624cd0c567e106a7d..6b21a01510f053c857912f0140ef6312b0b6b772 100644
--- a/webrtc/video/receive_statistics_proxy.h
+++ b/webrtc/video/receive_statistics_proxy.h
@@ -85,12 +85,12 @@ class ReceiveStatisticsProxy : public VCMReceiveStatisticsCallback,
   struct SampleCounter {
     SampleCounter() : sum(0), num_samples(0) {}
     void Add(int sample);
-    int Avg(int min_required_samples) const;
+    int Avg(int64_t min_required_samples) const;
     void Reset();
 
    private:
-    int sum;
-    int num_samples;
+    int64_t sum;
+    int64_t num_samples;
   };
   struct QpCounters {
     SampleCounter vp8;