[MediaRouter] Record DIAL device counts in DialMediaSinkServiceImpl

chrome/browser/media/router/media_router_metrics.h

Index: chrome/browser/media/router/media_router_metrics.h
diff --git a/chrome/browser/media/router/media_router_metrics.h b/chrome/browser/media/router/media_router_metrics.h
index ce3e2d252136b7f20ad80a43ff428df0372fa5a7..db20553982d503c304caac1eb2a9d7c7c5bebd1f 100644
--- a/chrome/browser/media/router/media_router_metrics.h
+++ b/chrome/browser/media/router/media_router_metrics.h
@@ -55,6 +55,8 @@ enum class MediaRouterUserAction {
 class MediaRouterMetrics {
  public:
   // UMA histogram names.
+  static const char kDialAvailableDeviceCount[];
+  static const char kDialKnownDeviceCount[];
   static const char kHistogramIconClickLocation[];
   static const char kHistogramUiDialogPaint[];
   static const char kHistogramUiDialogLoadedWithData[];
@@ -83,6 +85,10 @@ class MediaRouterMetrics {
   // Records the outcome in a create route response.
   static void RecordRouteCreationOutcome(
       MediaRouterRouteCreationOutcome outcome);
+
+  // Records device counts.
+  static void RecordDialDeviceCounts(size_t available_device_count,
+                                     size_t known_device_count);
 };
 
 }  // namespace media_router