Add a feature flag for the new Windows USB backend.

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index cb67503464991ba1d17769987acbcf269bef317a..f2ae7efcbd6394b1f175d44f007f1c76375761bc 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -71,6 +71,7 @@
 #include "content/public/common/content_switches.h"
 #include "content/public/common/feature_h264_with_openh264_ffmpeg.h"
 #include "content/public/common/features.h"
+#include "device/base/features.h"
 #include "extensions/features/features.h"
 #include "gin/public/gin_features.h"
 #include "media/audio/audio_features.h"
@@ -2208,7 +2209,13 @@ const FeatureEntry kFeatureEntries[] = {
     {"enable-midi-manager-dynamic-instantiation",
      IDS_FLAGS_ENABLE_MIDI_MANAGER_DYNAMIC_INSTANTIATION_NAME,
      IDS_FLAGS_ENABLE_MIDI_MANAGER_DYNAMIC_INSTANTIATION_DESCRIPTION, kOsAll,
-     FEATURE_VALUE_TYPE(midi::features::kMidiManagerDynamicInstantiation)}
+     FEATURE_VALUE_TYPE(midi::features::kMidiManagerDynamicInstantiation)},
+
+#if defined(OS_WIN)

[scheib, 2017/02/11 01:45:18]

Are these #if defined required? IIRC the logic of only showing the flag on the
right platforms is handled by the kOsWin flag that is passed in. I think this
may be cargo culting -- there are examples of other kOsAndroid that don't need
the guards.

[Reilly Grant (use Gerrit), 2017/02/11 01:48:55]

The base::Feature instance is defined within #if guards so I can't refer to it
here without them.

+    {"new-usb-backend", IDS_FLAGS_NEW_USB_BACKEND_NAME,
+     IDS_FLAGS_NEW_USB_BACKEND_DESCRIPTION, kOsWin,
+     FEATURE_VALUE_TYPE(device::kNewUsbBackend)},
+#endif  // defined(OS_WIN)
 
     // NOTE: Adding new command-line switches requires adding corresponding
     // entries to enum "LoginCustomFlags" in histograms.xml. See note in