Adds a new valid endpoint for a form's action. Can be either
a URI class or a simple string.

child-src lists the URLs for workers and embedded frame contents.
For example: child-src https://youtube.com would enable embedding
videos from YouTube but not from other origins.

@see http://www.w3.org/TR/CSP/#directive-child-src

@param string|array $uri
@param boolean|null $explicitReporting

@return $this