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}
blink::WebAppBannerClient* RenderFrameImpl::appBannerClient() {
  if (!app_banner_client_) {
    app_banner_client_ =
        GetContentClient()->renderer()->CreateAppBannerClient(this);
  }

  return app_banner_client_.get();
}

blink::WebAppBannerClient* RenderFrameImpl::appBannerClient() {
  if (!app_banner_client_) {
    app_banner_client_ =
        GetContentClient()->renderer()->CreateAppBannerClient(this);
  }

  return app_banner_client_.get();
}
