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::OnSelectPopupMenuItems(
    bool canceled,
    const std::vector<int>& selected_indices) {
  if (!external_popup_menu_)
    return;

  external_popup_menu_->DidSelectItems(canceled, selected_indices);
  external_popup_menu_.reset();
}

void RenderFrameImpl::OnSelectPopupMenuItems(
    bool canceled,
    const std::vector<int>& selected_indices) {
  if (!external_popup_menu_)
    return;

  external_popup_menu_->DidSelectItems(canceled, selected_indices);
  external_popup_menu_.reset();
}
