Fix for writeCapture2 to prevent DART Iframe/Javascript tags from being written twice.

The standard DART tag includes a script/noscript combo written inside of an iframe tag, for fallback to older browsers. (https://support.google.com/dfa/partner/answer/188733?hl=en)

This commit causes the contents of an iframe to be discarded in the same way noscript contents are discarded, so that the script tag written inside the iframe will not be buffered and written to the page.