For media element fetches, remove user:pass components always.

The compatible behavior for media element fetches is to ignore any
authorization information embedded in the URL. Insist on that by removing
the user and password components before starting the fetch.

Not doing so currently asserts in DocumentThreadableLoader::loadRequest(),
which expects that portion of the URL to have been processed already,
and removed. Satisfy that expectation here.

R=japhet@chromium.org,acolwell@chromium.org,philipj@opera.com
BUG=357269

Review URL: https://codereview.chromium.org/286993008

git-svn-id: svn://svn.chromium.org/blink/trunk@174248 bbb929c8-8fbe-4397-9dbb-9b2b20218538