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}
GURL RenderFrameImpl::GetLoadingUrl() const {
  WebDataSource* ds = frame_->dataSource();
  if (ds->hasUnreachableURL())
    return ds->unreachableURL();

  const WebURLRequest& request = ds->request();
  return request.url();
}

GURL RenderFrameImpl::GetLoadingUrl() const {
  WebDataSource* ds = frame_->dataSource();
  if (ds->hasUnreachableURL())
    return ds->unreachableURL();

  const WebURLRequest& request = ds->request();
  return request.url();
}
