Fetch API: Fix behavior when Request constructor is passed an undefined referrer

When the Request constructor was called with referrer set to undefined, the
resulting referrer used "undefined" as a string. After this code change,
an undefined referrer will be considered as not present and the default referrer
"about:client" will instead be used.

BUG=624278

Review-Url: https://codereview.chromium.org/2772013002
Cr-Commit-Position: refs/heads/master@{#460049}
10 files changed