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}
bool RenderFrameImpl::allowWebGL(blink::WebLocalFrame* frame,
                                 bool default_value) {
  DCHECK(!frame_ || frame_ == frame);
  if (!default_value)
    return false;

  bool blocked = true;
  render_view_->Send(new ViewHostMsg_Are3DAPIsBlocked(
      render_view_->GetRoutingID(),
      GURL(frame->top()->securityOrigin().toString()),
      THREE_D_API_TYPE_WEBGL,
      &blocked));
  return !blocked;
}

bool RenderFrameImpl::allowWebGL(blink::WebLocalFrame* frame,
                                 bool default_value) {
  DCHECK(!frame_ || frame_ == frame);
  if (!default_value)
    return false;

  bool blocked = true;
  render_view_->Send(new ViewHostMsg_Are3DAPIsBlocked(
      render_view_->GetRoutingID(),
      GURL(frame->top()->securityOrigin().toString()),
      THREE_D_API_TYPE_WEBGL,
      &blocked));
  return !blocked;
}
