Adding histogram for API call jitter in WebRTC/AEC3

This CL adds histograms to monitor the API call jitter that is observed
in AEC3 in WebRTC.

The WebRTC CL for adding the histograms is
https://webrtc-review.googlesource.com/c/src/+/111580

The metrics for the API call jitter is the minimum and maximum
calls API calls in a row (in units blocks of 8ms) observed for
capture and render.

Bug: chromium:907234
Change-Id: I7f6d45b9f5826f6f7706a6ec3644ec0c35a3da00
Reviewed-on: https://chromium-review.googlesource.com/c/1345012
Commit-Queue: Per Åhgren <peah@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611600}

tools/metrics/histograms/histograms.xml

@@ -123313,6 +123313,26 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.Audio.EchoCanceller.MaxCaptureJitter"
+    units="frames (10 ms)" expires_after="2019-05-01">
+  <owner>peah@chromium.org</owner>
+  <owner>gustaf@chromium.org</owner>
+  <summary>
+    This histogram logs the observed maximum number of capture API calls in a
+    row in the unit of frames (10 ms). A new value is logged every 10 seconds.
+  </summary>
+</histogram>
+
+<histogram name="WebRTC.Audio.EchoCanceller.MaxRenderJitter"
+    units="frames (10 ms)" expires_after="2019-05-01">
+  <owner>peah@chromium.org</owner>
+  <owner>gustaf@chromium.org</owner>
+  <summary>
+    This histogram logs the observed maximum number of render API calls in a row
+    in the unit of frames (10 ms). A new value is logged every 10 seconds.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.Audio.EchoCanceller.MaxSkewShiftCount"
     units="events/minute">
   <owner>peah@chromium.org</owner>
@@ -123329,6 +123349,26 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="WebRTC.Audio.EchoCanceller.MinCaptureJitter"
+    units="frames (10 ms)" expires_after="2019-05-01">
+  <owner>peah@chromium.org</owner>
+  <owner>gustaf@chromium.org</owner>
+  <summary>
+    This histogram logs the observed minimum number of capture API calls in a
+    row in the unit of frames (10 ms). A new value is logged every 10 seconds.
+  </summary>
+</histogram>
+
+<histogram name="WebRTC.Audio.EchoCanceller.MinRenderJitter"
+    units="frames (10 ms)" expires_after="2019-05-01">
+  <owner>peah@chromium.org</owner>
+  <owner>gustaf@chromium.org</owner>
+  <summary>
+    This histogram logs the observed minimum number of render API calls in a row
+    in the unit of frames (10 ms). A new value is logged every 10 seconds.
+  </summary>
+</histogram>
+
 <histogram name="WebRTC.Audio.EchoCanceller.ModelBasedAecFeasible"
     enum="Boolean">
   <owner>peah@chromium.org</owner>