Log device/usb messages to chrome://device-log.

device/usb/usb_service.cc

Index: device/usb/usb_service.cc
diff --git a/device/usb/usb_service.cc b/device/usb/usb_service.cc
index 3faa3e56413a24335ca1fde440b41ae4657f410b..5bb7f20f2e7c2d151cfaa7c4b5804e08d4fbc946 100644
--- a/device/usb/usb_service.cc
+++ b/device/usb/usb_service.cc
@@ -6,6 +6,7 @@
 
 #include "base/message_loop/message_loop.h"
 #include "device/usb/usb_device.h"
+#include "device/usb/usb_log.h"
 #include "device/usb/usb_service_impl.h"
 
 namespace device {
@@ -83,11 +84,19 @@ void UsbService::RemoveObserver(Observer* observer) {
 
 void UsbService::NotifyDeviceAdded(scoped_refptr<UsbDevice> device) {
   DCHECK(CalledOnValidThread());
+
+  USB_LOG(EVENT) << "USB device added: vendorId = " << device->vendor_id()
+                 << ", productId = " << device->product_id()
+                 << ", uniqueId = " << device->unique_id();

[stevenjb, 2015/02/20 00:01:27]

These might be a good candidate for 'USER' since it might represent a user
inserting or removing a device. Even if some devices are built in and not
physically added I think that would be OK. It's a convenient way to identify
"significant" events that are likely to drive other events.

[Reilly Grant (use Gerrit), 2015/02/20 00:10:22]

Fair enough. There will be a small burst of events when the service is
initialized but otherwise this should be mostly user-driven.

+
   FOR_EACH_OBSERVER(Observer, observer_list_, OnDeviceAdded(device));
 }
 
 void UsbService::NotifyDeviceRemoved(scoped_refptr<UsbDevice> device) {
   DCHECK(CalledOnValidThread());
+
+  USB_LOG(EVENT) << "USB device removed: uniqueId = " << device->unique_id();
+
   FOR_EACH_OBSERVER(Observer, observer_list_, OnDeviceRemoved(device));
   FOR_EACH_OBSERVER(Observer, observer_list_, OnDeviceRemovedCleanup(device));
 }