Use xmlnsWithColon instead of literal

We can just use the existing global xmlnsWithColon
and avoid a potential strlen on the literal.

Review URL: https://codereview.chromium.org/590193002

git-svn-id: svn://svn.chromium.org/blink/trunk@182444 bbb929c8-8fbe-4397-9dbb-9b2b20218538