[Media Router] Add MediaSink subtypes

chrome/browser/media/router/media_sink.h

Index: chrome/browser/media/router/media_sink.h
diff --git a/chrome/browser/media/router/media_sink.h b/chrome/browser/media/router/media_sink.h
index e348fe21c55977b2412473deb669c94449457164..f5ef6558041c59a0c3bdce21bcaf93c8e18880ab 100644
--- a/chrome/browser/media/router/media_sink.h
+++ b/chrome/browser/media/router/media_sink.h
@@ -17,6 +17,7 @@ class Collator;
 namespace media_router {
 
 // Represents a sink to which media can be routed.
+// TODO(zhaobin): convert MediaSink into a struct.
 class MediaSink {
  public:
   using Id = std::string;
@@ -59,6 +60,7 @@ class MediaSink {
   void set_icon_type(IconType icon_type) { icon_type_ = icon_type; }
   IconType icon_type() const { return icon_type_; }
 
+  // This method only compares IDs.
   bool Equals(const MediaSink& other) const;
 
   // Compares |this| to |other| first by their icon types, then their names