Rename USB FIDO transport to “usb”.

This matches the name in the spec[1]. However, I note that these strings
might be saved in preferences now, so we might be too late here. If so,
we can always create another enum for this.

[1] https://w3c.github.io/webauthn/#enumdef-authenticatortransport

Change-Id: Id74e1d62a72d85b0bd96c0996d4a804de5911531
Reviewed-on: https://chromium-review.googlesource.com/1176781Reviewed-by: Jun Choi <hongjunchoi@chromium.org>
Commit-Queue: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583506}

device/fido/fido_transport_protocol.cc

@@ -6,7 +6,7 @@
 
 namespace device {
 
-const char kUsbHumanInterfaceDevice[] = "hid";
+const char kUsbHumanInterfaceDevice[] = "usb";
 const char kNearFieldCommunication[] = "nfc";
 const char kBluetoothLowEnergy[] = "ble";
 const char kCloudAssistedBluetoothLowEnergy[] = "cable";