UMA histogram for tracking level of clockdrift in the echo canceller

Enable tracking of the level of clockdrift seen by the WebRTC acoustic
echo canceller. The histogram has three bins:
0: No clockdrift detected.
1: Clockdrift probable.
2: Clockdrift detected.

Bug: webrtc:10014
Change-Id: Ide08da65177cc0fe32c96fcad19d87d6ab8c7dc5
Reviewed-on: https://chromium-review.googlesource.com/c/1341919Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Gustaf Ullberg <gustaf@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610797}

tools/metrics/histograms/enums.xml

@@ -7065,6 +7065,12 @@ histogram as enum -->
   <int value="2" label="Read Text"/>
 </enum>
 
+<enum name="ClockdriftLevel">
+  <int value="0" label="None"/>
+  <int value="1" label="Probable"/>
+  <int value="2" label="Verified"/>
+</enum>
+
 <enum name="ClockStates">
   <int value="0"
       label="CLOCK_STATE_UNKNOWN: accuracy of system clock is unknown"/>

tools/metrics/histograms/histograms.xml

@@ -122836,6 +122836,15 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.Audio.EchoCanceller.Clockdrift" enum="ClockdriftLevel"
+    expires_after="2019-11-30">
+  <owner>gustaf@chromium.org</owner>
+  <summary>
+    This histogram logs whether clockdrift is detected in the WebRTC echo
+    canceller. A new value is logged every 10 seconds.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.Audio.EchoCanceller.ComfortNoiseBand0"
     units="dBFS (negated)">
   <owner>peah@chromium.org</owner>