Add webrtc metrics for video pause events to histograms.xml.

Metric added in: https://codereview.webrtc.org/2530393003/

BUG=

Review-Url: https://codereview.chromium.org/2540323006
Cr-Commit-Position: refs/heads/master@{#439463}

tools/metrics/histograms/histograms.xml

@@ -74082,6 +74082,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.Video.NumberOfPauseEvents" units="pause events">
+  <owner>asapersson@chromium.org</owner>
+  <summary>
+    The number of times a video stream has been paused/resumed during a call.
+    Recorded when a stream is removed.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.Video.OnewayDelayInMs" units="ms">
   <owner>asapersson@chromium.org</owner>
   <summary>
@@ -74108,6 +74116,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.Video.PausedTimeInPercent" units="%">
+  <owner>asapersson@chromium.org</owner>
+  <summary>
+    Percentage of time that the video has been paused for a sent video stream.
+    Recorded when a stream is removed.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.Video.PliPacketsReceivedPerMinute"
     units="packets/minute">
   <owner>asapersson@chromium.org</owner>