Expose video freeze metrics in GetStats.

This adds the following non-standardized metrics to video receiver
stats:
- freezeCount
- pauseCount
- totalFreezesDuration
- totalPausesDuration
- totalFramesDuration
- sumOfSquaredFrameDurations

For description of these metrics see
https://henbos.github.io/webrtc-provisional-stats/#RTCVideoReceiverStats-dict*

Bug: webrtc:10145
Change-Id: I4c76d5651102e73b1592ffed561e6224f2badeb6
Reviewed-on: https://webrtc-review.googlesource.com/c/114840
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26523}

rtc_base/numerics/sample_counter_unittest.cc

@@ -33,6 +33,7 @@ TEST(SampleCounterTest, NotEnoughSamples) {
     counter.Add(value);
   }
   EXPECT_THAT(counter.Avg(kMinSamples), Eq(absl::nullopt));
+  EXPECT_THAT(counter.Sum(kMinSamples), Eq(absl::nullopt));
   EXPECT_THAT(counter.Max(), Eq(5));
 }
 
@@ -43,6 +44,7 @@ TEST(SampleCounterTest, EnoughSamples) {
     counter.Add(value);
   }
   EXPECT_THAT(counter.Avg(kMinSamples), Eq(3));
+  EXPECT_THAT(counter.Sum(kMinSamples), Eq(15));
   EXPECT_THAT(counter.Max(), Eq(5));
 }