Remove zero-divide in VCMContentMetricsProcessing.

Protects against zero-framerate in UpdateFrameRate.

BUG=webrtc:5124
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1636943003 .

Cr-Commit-Position: refs/heads/master@{#11382}

webrtc/modules/video_coding/content_metrics_processing.cc

@@ -44,6 +44,8 @@ int VCMContentMetricsProcessing::Reset() {
 }
 
 void VCMContentMetricsProcessing::UpdateFrameRate(uint32_t frameRate) {
+  if (frameRate == 0)
+    frameRate = 1;
   // Update factor for recursive averaging.
   recursive_avg_factor_ = static_cast<float>(1000.0f) /
                           static_cast<float>(frameRate * kQmMinIntervalMs);