extensions: Explicitly ignore result of CalledOnValidThread().

BUG=314505

Review URL: https://codereview.chromium.org/597853003

Cr-Commit-Position: refs/heads/master@{#296640}

extensions/browser/api/hid/hid_device_manager.cc

@@ -143,7 +143,7 @@ bool HidDeviceManager::HasPermission(const Extension* extension,
 }
 
 void HidDeviceManager::UpdateDevices() {
-  thread_checker_.CalledOnValidThread();
+  DCHECK(thread_checker_.CalledOnValidThread());
   HidService* hid_service = device::DeviceClient::Get()->GetHidService();
   DCHECK(hid_service);