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::didChangeSandboxFlags(blink::WebFrame* child_frame,
                                            blink::WebSandboxFlags flags) {
  int frame_routing_id = MSG_ROUTING_NONE;
  if (child_frame->isWebRemoteFrame()) {
    frame_routing_id =
        RenderFrameProxy::FromWebFrame(child_frame)->routing_id();
  } else {
    frame_routing_id =
        RenderFrameImpl::FromWebFrame(child_frame)->GetRoutingID();
  }

  Send(new FrameHostMsg_DidChangeSandboxFlags(routing_id_, frame_routing_id,
                                              flags));
}

void RenderFrameImpl::didChangeSandboxFlags(blink::WebFrame* child_frame,
                                            blink::WebSandboxFlags flags) {
  int frame_routing_id = MSG_ROUTING_NONE;
  if (child_frame->isWebRemoteFrame()) {
    frame_routing_id =
        RenderFrameProxy::FromWebFrame(child_frame)->routing_id();
  } else {
    frame_routing_id =
        RenderFrameImpl::FromWebFrame(child_frame)->GetRoutingID();
  }

  Send(new FrameHostMsg_DidChangeSandboxFlags(routing_id_, frame_routing_id,
                                              flags));
}
