Deal with Parsoid's protocol-relative <base> tags in a DOMParser world

Protocol-relative <base> tags are fine when the iframe trick is used
because the iframe inherits a default base URL from the parent document,
so if the <base> is relative it can be resolved against that. However,
a document created with DOMParser (or document.implementation, oops!)
won't inherit anything, so anything other than a fully absolute <base>
will just break.

To make Parsoid's relative <base> work anyway, resolve it in the
context of the main document, then write the resolved version
back into the <base> tag.

Change-Id: I7261bd5dbe9ec1f4077a4e5a00bd2a03ac237ca0