Mark metrics obsolete due to AEC2 deprecation.

The acoustic echo canceler AEC2 is deprecated. There are no existing Chrome clients reporting WebRTC.AecDelayBasedQuality.

Bug: webrtc:10563
Change-Id: I10e6cd376524564fd1b355b34d510e376ecb4dc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609240
Auto-Submit: Sam Zackrisson <saza@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659349}

tools/metrics/histograms/histograms.xml

@@ -140073,6 +140073,10 @@ should be kept until we use this API.  -->
 </histogram>
 
 <histogram name="WebRTC.AecDelayBasedQuality" enum="DelayBasedEchoQuality">
+  <obsolete>
+    Deprecated 5/2019 in issue bugs.webrtc.org/10563 due to the reporting
+    component (AEC2) being deprecated.
+  </obsolete>
   <owner>hlundin@chromium.org</owner>
   <summary>
     Captures if the estimated delay between rendered and captured audio is out