[Media] Record time it takes to start rendering audio and video

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 052076318d4908d6ed69a3c6511c904884fa7bb9..c26d1dc4f12647726830eb7705d3a1fb4664baa1 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -23813,6 +23813,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.Audio.PrerollTime" units="ms">
+  <owner>avayvod@chromium.org</owner>
+  <owner>dalecurtis@chromium.org</owner>
+  <summary>
+    Records the time between the moment when the audio is requested to start
+    from a new position and when there's enough data decoded to start playing.
+    Recorded every time there's enough data buffered after playback is initiated
+    (on initial play or after seeking). Recorded for both audio only streams and
+    video streams with audio.
+  </summary>
+</histogram>
+
 <histogram name="Media.Audio.Render.AudioInputsPerMixer.LatencyExact"
     units="inputs">
   <owner>olka@chromium.org</owner>
@@ -25493,6 +25505,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.Video.PrerollTime" units="ms">
+  <owner>avayvod@chromium.org</owner>
+  <owner>dalecurtis@chromium.org</owner>
+  <summary>
+    Records the time between the moment when the video is requested to start
+    from a new position and when the video renderer has enough decoded data to
+    start painting. Recorded every time there's enough data after playback is
+    initiated (on initial play or after seeking).
+  </summary>
+</histogram>
+
 <histogram name="Media.VideoCapture.AspectRatio" units="%">
   <owner>mcasas@chromium.org</owner>
   <summary>