Manage UsbService lifetime in DeviceClient implementations.

chrome/browser/chrome_device_client.h

Index: chrome/browser/chrome_device_client.h
diff --git a/chrome/browser/chrome_device_client.h b/chrome/browser/chrome_device_client.h
index 1cd401f7a9dc7476b958e8ea2d480c20afdb4bbc..f8db0f2dcd21ff92ace5e8b9f52a4a46f9bb0c00 100644
--- a/chrome/browser/chrome_device_client.h
+++ b/chrome/browser/chrome_device_client.h
@@ -9,6 +9,7 @@
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
+#include "base/memory/scoped_ptr.h"
 
 // Implementation of device::DeviceClient that returns //device service
 // singletons appropriate for use within the Chrome application.
@@ -22,6 +23,8 @@ class ChromeDeviceClient : device::DeviceClient {
   device::HidService* GetHidService() override;
 
  private:
+  scoped_ptr<device::UsbService> usb_service_;
+
   DISALLOW_COPY_AND_ASSIGN(ChromeDeviceClient);
 };