Add WebUSB bindings and client interface

public/web/WebFrameClient.h

Index: public/web/WebFrameClient.h
diff --git a/public/web/WebFrameClient.h b/public/web/WebFrameClient.h
index 09dffc4ba2292791d6c157ec5fe8ed3f1949e8f3..457d78cbd391daab3c422caabe315bb5287f8852 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 WebWorkerContentSettingsClientProxy;
@@ -665,6 +666,9 @@ public:
     // Bluetooth -----------------------------------------------------------
     virtual WebBluetooth* bluetooth() { return 0; }
 
+    // WebUSB --------------------------------------------------------------
+    virtual WebUSBClient* usbClient() { return 0; }
+
 protected:
     virtual ~WebFrameClient() { }
 };