Add test against crashes in VideoQualityObserver

If there were a lot of pauses in the receive video stream, it may've
caused a crash because of a null rtc::Optional dereferencing.

This is the test reproducing that behaviour.

Bug: webrtc:9338
Change-Id: I1cef72a88a54f762ef27665d372e4a1d1225e059
Reviewed-on: https://webrtc-review.googlesource.com/80161
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23470}

video/receive_statistics_proxy_unittest.cc

@@ -1046,6 +1046,33 @@ TEST_P(ReceiveStatisticsProxyTest, PausesAreIgnored) {
   }
 }
 
+TEST_P(ReceiveStatisticsProxyTest, ManyPausesAtTheBeginning) {
+  const VideoContentType content_type = GetParam();
+  const int kInterFrameDelayMs = 33;
+  const int kPauseDurationMs = 10000;
+  for (int i = 0; i <= kMinRequiredSamples; ++i) {
+    statistics_proxy_->OnDecodedFrame(rtc::nullopt, kWidth, kHeight,
+                                      content_type);
+    fake_clock_.AdvanceTimeMilliseconds(kInterFrameDelayMs);
+
+    statistics_proxy_->OnStreamInactive();
+    fake_clock_.AdvanceTimeMilliseconds(kPauseDurationMs);
+
+    statistics_proxy_->OnDecodedFrame(rtc::nullopt, kWidth, kHeight,
+                                          content_type);
+    fake_clock_.AdvanceTimeMilliseconds(kInterFrameDelayMs);
+  }
+
+  statistics_proxy_.reset();
+  // No freezes should be detected, as all long inter-frame delays were pauses.
+  if (videocontenttypehelpers::IsScreenshare(content_type)) {
+    EXPECT_EQ(-1, metrics::MinSample(
+                      "WebRTC.Video.Screenshare.MeanFreezeDurationMs"));
+  } else {
+    EXPECT_EQ(-1, metrics::MinSample("WebRTC.Video.MeanFreezeDurationMs"));
+  }
+}
+
 TEST_P(ReceiveStatisticsProxyTest, TimeInHdReported) {
   const VideoContentType content_type = GetParam();
   const int kInterFrameDelayMs = 20;