Add WebUSB bindings and client interface [part 1]

public/web/WebFrameClient.h

Index: public/web/WebFrameClient.h
diff --git a/public/web/WebFrameClient.h b/public/web/WebFrameClient.h
index 0092a6c805fe84904a4bb3bc876c9317f57b300c..ce2106a8a91a53befc6f9927e1d9ba20a6508cde 100644
--- a/public/web/WebFrameClient.h
+++ b/public/web/WebFrameClient.h
@@ -89,6 +89,7 @@ class WebScreenOrientationClient;
 class WebString;
 class WebURL;
 class WebURLResponse;
+class WebUSBClient;
 class WebUserMediaClient;
 class WebVRClient;
 class WebWakeLockClient;
@@ -669,6 +670,9 @@ public:
     // Bluetooth -----------------------------------------------------------
     virtual WebBluetooth* bluetooth() { return 0; }
 
+    // WebUSB --------------------------------------------------------------
+    virtual WebUSBClient* usbClient() { return nullptr; }
+
 protected:
     virtual ~WebFrameClient() { }
 };