Add UMA for the number of devices in the chooser when a device is paired

content/browser/bluetooth/bluetooth_device_chooser_controller.h

Index: content/browser/bluetooth/bluetooth_device_chooser_controller.h
diff --git a/content/browser/bluetooth/bluetooth_device_chooser_controller.h b/content/browser/bluetooth/bluetooth_device_chooser_controller.h
index 500d31b144c125243d9ff0417a2b8e8a25aa9511..a054d6bf7058b521c3ecfde6c72256fd88122698 100644
--- a/content/browser/bluetooth/bluetooth_device_chooser_controller.h
+++ b/content/browser/bluetooth/bluetooth_device_chooser_controller.h
@@ -6,6 +6,7 @@
 #define CONTENT_BROWSER_BLUETOOTH_BLUETOOTH_DEVICE_CHOOSER_CONTROLLER_H_
 
 #include <string>
+#include <unordered_set>
 
 #include "base/memory/weak_ptr.h"
 #include "base/optional.h"
@@ -149,6 +150,9 @@ class CONTENT_EXPORT BluetoothDeviceChooserController final {
   // The time when scanning starts.
   base::Optional<base::TimeTicks> scanning_start_time_;
 
+  // The device ids that are currently shown in the chooser.
+  std::unordered_set<std::string> device_ids_;
+
   // Weak pointer factory for generating 'this' pointers that might live longer
   // than we do.
   // Note: This should remain the last member so it'll be destroyed and