Don't emit a default namespace declaration if it's already the default

If an element declares a new default namespace (has an xmlns="..."
attribute) make sure said namespace is used to update the namespace-table.
This avoids having to emit a default namespace declaration for descendants
of said element.

BUG=340794

Review URL: https://codereview.chromium.org/184633007

git-svn-id: svn://svn.chromium.org/blink/trunk@168672 bbb929c8-8fbe-4397-9dbb-9b2b20218538