Destroy DeviceImpl when the underlying UsbDevice is disconnected.

device/usb/usb_device.h

Index: device/usb/usb_device.h
diff --git a/device/usb/usb_device.h b/device/usb/usb_device.h
index 27454051a010e113b3c3e15766e2da5a587a0f0e..b1406d9b23be76c37264c6dc8b800480fabe6f6b 100644
--- a/device/usb/usb_device.h
+++ b/device/usb/usb_device.h
@@ -13,6 +13,7 @@
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/observer_list.h"
 #include "base/strings/string16.h"
 #include "device/usb/usb_descriptors.h"
 #include "url/gurl.h"
@@ -32,6 +33,18 @@ class UsbDevice : public base::RefCountedThreadSafe<UsbDevice> {
   using OpenCallback = base::Callback<void(scoped_refptr<UsbDeviceHandle>)>;
   using ResultCallback = base::Callback<void(bool success)>;
 
+  // This observer interface should be used by objects that need only be
+  // notified about the removal of a particular device as it is more efficient
+  // than registering a large number of observers with UsbService::AddObserver.
+  class Observer {
+   public:
+    virtual ~Observer();
+
+    // This method is called when the UsbService that created this object
+    // detects that the device has been disconnected from the host.
+    virtual void OnDeviceRemoved(scoped_refptr<UsbDevice> device);
+  };
+
   // A unique identifier which remains stable for the lifetime of this device
   // object (i.e., until the device is unplugged or the USB service dies.)
   const std::string& guid() const { return guid_; }
@@ -64,7 +77,12 @@ class UsbDevice : public base::RefCountedThreadSafe<UsbDevice> {
   // if the device is unconfigured.
   virtual const UsbConfigDescriptor* GetActiveConfiguration() = 0;
 
+  void AddObserver(Observer* observer);
+  void RemoveObserver(Observer* observer);
+
  protected:
+  friend class UsbService;
+
   UsbDevice(uint16_t vendor_id,
             uint16_t product_id,
             const base::string16& manufacturer_string,
@@ -72,6 +90,8 @@ class UsbDevice : public base::RefCountedThreadSafe<UsbDevice> {
             const base::string16& serial_number);
   virtual ~UsbDevice();
 
+  void NotifyDeviceRemoved();
+
   // These members must be mutable by subclasses as necessary during device
   // enumeration. To preserve the thread safety of this object they must remain
   // constant afterwards.
@@ -91,6 +111,8 @@ class UsbDevice : public base::RefCountedThreadSafe<UsbDevice> {
   const uint16_t vendor_id_;
   const uint16_t product_id_;
 
+  base::ObserverList<Observer, true> observer_list_;
+
   DISALLOW_COPY_AND_ASSIGN(UsbDevice);
 };