Connect WebUSB client interface to the devices app

This provides a basic WebUSB client interface in
content/renderer. Most of the interface is unimplemented,
but this CL hooks up navigator.usb.getDevices() to the
browser's Mojo devices app to enumerate available USB
devices.

BUG=492204

Review URL: https://codereview.chromium.org/1293253002

Cr-Commit-Position: refs/heads/master@{#344881}
void RenderFrameImpl::registerProtocolHandler(const WebString& scheme,
                                              const WebURL& url,
                                              const WebString& title) {
  bool user_gesture = WebUserGestureIndicator::isProcessingUserGesture();
  Send(new FrameHostMsg_RegisterProtocolHandler(
      routing_id_,
      base::UTF16ToUTF8(base::StringPiece16(scheme)),
      url,
      title,
      user_gesture));
}

void RenderFrameImpl::registerProtocolHandler(const WebString& scheme,
                                              const WebURL& url,
                                              const WebString& title) {
  bool user_gesture = WebUserGestureIndicator::isProcessingUserGesture();
  Send(new FrameHostMsg_RegisterProtocolHandler(
      routing_id_,
      base::UTF16ToUTF8(base::StringPiece16(scheme)),
      url,
      title,
      user_gesture));
}
