Do not add sub-resources for data url in MHTML serialization

 data url are self-contained, and adding sub resources for those will
serialize the content 3 times for no gain.

R=abarth@chromium.org, thakis@chromium.org

Review URL: https://codereview.chromium.org/19757002

git-svn-id: svn://svn.chromium.org/blink/trunk@154642 bbb929c8-8fbe-4397-9dbb-9b2b20218538