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::requestNotificationPermission(
    const blink::WebSecurityOrigin& origin,
    blink::WebNotificationPermissionCallback* callback) {
  if (!notification_permission_dispatcher_) {
    notification_permission_dispatcher_ =
        new NotificationPermissionDispatcher(this);
  }

  notification_permission_dispatcher_->RequestPermission(origin, callback);
}

void RenderFrameImpl::requestNotificationPermission(
    const blink::WebSecurityOrigin& origin,
    blink::WebNotificationPermissionCallback* callback) {
  if (!notification_permission_dispatcher_) {
    notification_permission_dispatcher_ =
        new NotificationPermissionDispatcher(this);
  }

  notification_permission_dispatcher_->RequestPermission(origin, callback);
}
