Remove DCHECK on device removal

DCHECK on FidoRequestHandlerBase::OnDeviceRemoved will always fail if
user removes a device that has already been removed due to processing
error.

Bug: 780078
Change-Id: Ica0ee018645936706f1691378e9c7f0ad3fd0ca4
Reviewed-on: https://chromium-review.googlesource.com/1006492
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550139}

device/fido/fido_request_handler_base.cc

@@ -62,8 +62,10 @@ void FidoRequestHandlerBase::DeviceAdded(FidoDiscovery* discovery,
 void FidoRequestHandlerBase::DeviceRemoved(FidoDiscovery* discovery,
                                            FidoDevice* device) {
   // Device connection has been lost or device has already been removed.
-  // Thus, calling CancelTask() is not necessary.
-  DCHECK(base::ContainsKey(ongoing_tasks_, device->GetId()));
+  // Thus, calling CancelTask() is not necessary. Also, below
+  // ongoing_tasks_.erase() will have no effect for the devices that have been
+  // already removed due to processing error or due to invocation of
+  // CancelOngoingTasks().
   ongoing_tasks_.erase(device->GetId());
 }