Add "use-winrt-midi-api" flag to enable WinRT WebMIDI backend

media/midi/BUILD.gn

Index: media/midi/BUILD.gn
diff --git a/media/midi/BUILD.gn b/media/midi/BUILD.gn
index f3758563f3b65f7ff5e2d29154b3dc5e4e4710f1..cdea3b77f6ae812369d94a515bf80c66e50b811a 100644
--- a/media/midi/BUILD.gn
+++ b/media/midi/BUILD.gn
@@ -6,7 +6,6 @@ import("//build/config/android/config.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//media/media_options.gni")
-import("//media/midi/midi_options.gni")
 import("//testing/test.gni")
 
 if (is_android) {
@@ -137,18 +136,13 @@ component("midi") {
   }
 
   if (is_win) {
-    if (midi_use_winrt) {
-      sources += [
-        "midi_manager_winrt.cc",
-        "midi_manager_winrt.h",
-      ]
-    } else {
-      deps += [ "//device/usb" ]
-      sources += [
-        "midi_manager_win.cc",
-        "midi_manager_win.h",
-      ]
-    }
+    deps += [ "//device/usb" ]
+    sources += [
+      "midi_manager_win.cc",
+      "midi_manager_win.h",
+      "midi_manager_winrt.cc",
+      "midi_manager_winrt.h",
+    ]
   }
 
   if (use_alsa && use_udev) {