Allow <link rel=stylesheet> in a connected shadow tree

See https://github.com/w3c/webcomponents/issues/530. We have relaxed the condition.

"Web-Facing Change PSA" mail in blink-dev is:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/b7g62d2yqWo/2itpAFafBgAJ

Regarding a browsing context, this CL does not change any behavior. That should be
another concern and should be investigated later.

See also https://github.com/w3c/webcomponents/issues/535. "title= attribute" is
ignored completely for <link rel=stylesheet>.

BUG=630141

Review-Url: https://codereview.chromium.org/2177163002
Cr-Commit-Position: refs/heads/master@{#409728}
9 files changed