Allow filename suggestions via a[download] for data URIs

Currently filename suggestions specified via a[download] are only honored if
the interface origin is allowed to read content retrieved from the target
resource origin. An embedder may enforce additional restrictions such as only
honoring the suggested name if there are no cross-origin redirects encountered
while fetching the resource.

The suggested filename determination algorithm at
http://www.w3.org/TR/html5/links.html#downloading-resources allows an exception
for data URIs. They should be considered same-origin as the interface. This
isn't currently the case since the origin of a data URI is considerd to be
unique and is not same-origin with anything since they lack a server-based
naming authority.

This CL implements the exception for data URIs so that they are considered
same-origin as their containing document for the purpose of handling the
suggested filename for a[download].

BUG=373182

Review URL: https://codereview.chromium.org/300543002

git-svn-id: svn://svn.chromium.org/blink/trunk@176085 bbb929c8-8fbe-4397-9dbb-9b2b20218538