Track how many times a video capture device is accessed simultaneously.

This CL tracks the number of times a video capture device is accessed
simultaneously to understand if Chrome should add some restrictions
around simultaneous camera access.

Test: Go to chrome://histrograms/Media.VideoCapture.NumberOfClients
and access camera device from several tabs.

Change-Id: I1a4bb4020ff13bd1ddc54a59094379115e2f65e5
Bug: 1134077
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2440514
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814600}

content/browser/renderer_host/media/video_capture_controller.cc

@@ -333,6 +333,8 @@ void VideoCaptureController::AddClient(
   // client.
   if (state_ != blink::VIDEO_CAPTURE_STATE_ERROR) {
     controller_clients_.push_back(std::move(client));
+    base::UmaHistogramCounts100("Media.VideoCapture.NumberOfClients",
+                                controller_clients_.size());
   }
 }
 

tools/metrics/histograms/histograms_xml/media/histograms.xml

@@ -3626,6 +3626,16 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="Media.VideoCapture.NumberOfClients" units="units"
+    expires_after="M95">
+  <owner>fbeaufort@chromium.org</owner>
+  <owner>guidou@chromium.org</owner>
+  <summary>
+    Number of times a video capture device is accessed simultaneously. Recorded
+    when video capturing starts.
+  </summary>
+</histogram>
+
 <histogram name="Media.VideoCapture.Width" units="pixels" expires_after="M82">
   <owner>mcasas@chromium.org</owner>
   <summary>