Call libusb_init on the FILE thread.

device/usb/usb_service_impl.h

Index: device/usb/usb_service_impl.h
diff --git a/device/usb/usb_service_impl.h b/device/usb/usb_service_impl.h
index ddca3b5bffc1bf344ab26a91c8da422799ab1922..e5467fb0b0c4ea99fcc2fd7b575484dba08da4fa 100644
--- a/device/usb/usb_service_impl.h
+++ b/device/usb/usb_service_impl.h
@@ -56,6 +56,8 @@ class UsbServiceImpl :
                        const std::string& device_path) override;
 #endif  // OS_WIN
 
+  void OnUsbContext(scoped_refptr<UsbContext> context);
+
   // Enumerate USB devices from OS and update devices_ map.
   void RefreshDevices();
   void OnDeviceList(libusb_device** platform_devices, size_t device_count);
@@ -84,6 +86,7 @@ class UsbServiceImpl :
                          const base::Closure& refresh_complete);
 
   scoped_refptr<UsbContext> context_;
+  bool usb_unavailable_ = false;

[juncai, 2016/12/08 20:00:39]

nit: maybe change the variable name to be:
bool usb_available_ = true;

[Reilly Grant (use Gerrit), 2016/12/08 22:44:39]

The reason I named this usb_unavailable_ is because until we see if we can
initialize context_ we don't know if USB will be available or not, so it seemed
less surprising to have a variable that becomes true only when we've given up on
initializing USB than a variable that it true before we've even tried.

[juncai, 2016/12/08 23:02:10]

I see. Thanks for the explanation.

 
   // When available the device list will be updated when new devices are
   // connected instead of only when a full enumeration is requested.