webusb: Reject the promise if the device is already open.

device/usb/mojo/device_impl.cc

Index: device/usb/mojo/device_impl.cc
diff --git a/device/usb/mojo/device_impl.cc b/device/usb/mojo/device_impl.cc
index 724e911d2977df81160944290c65ba18c2056bb5..0d1c64f6a2dd1336fe8b659ce9407633af9dbdf7 100644
--- a/device/usb/mojo/device_impl.cc
+++ b/device/usb/mojo/device_impl.cc
@@ -251,8 +251,11 @@ void DeviceImpl::GetConfiguration(const GetConfigurationCallback& callback) {
 }
 
 void DeviceImpl::Open(const OpenCallback& callback) {
-  device_->Open(
-      base::Bind(&DeviceImpl::OnOpen, weak_factory_.GetWeakPtr(), callback));
+  if (device_handle_)
+    callback.Run(OpenDeviceError::ALREADY_OPEN);
+  else
+    device_->Open(
+        base::Bind(&DeviceImpl::OnOpen, weak_factory_.GetWeakPtr(), callback));
 }
 
 void DeviceImpl::Close(const CloseCallback& callback) {