Parse USB interface association descriptors.

device/usb/usb_descriptors.h

Index: device/usb/usb_descriptors.h
diff --git a/device/usb/usb_descriptors.h b/device/usb/usb_descriptors.h
index c99a6def7bf9e3e37c1849517e572bc692a895fe..a1414154ff757eea25b14b20b967a2b3ded761d3 100644
--- a/device/usb/usb_descriptors.h
+++ b/device/usb/usb_descriptors.h
@@ -85,6 +85,8 @@ struct UsbInterfaceDescriptor {
   uint8_t interface_protocol;
   std::vector<UsbEndpointDescriptor> endpoints;
   std::vector<uint8_t> extra_data;
+  // First interface of the function to which this interface belongs.
+  uint8_t first_interface;
 };
 
 struct UsbConfigDescriptor {
@@ -95,6 +97,10 @@ struct UsbConfigDescriptor {
   UsbConfigDescriptor() = delete;
   ~UsbConfigDescriptor();
 
+  // Scans through |extra_data| for interface association descriptors and
+  // populates |first_interface| for each interface in this configuration.
+  void AssignFirstInterfaceNumbers();
+
   uint8_t configuration_value;
   bool self_powered;
   bool remote_wakeup;