Avoid crash when setting rel=stylesheet on <link> in shadow root.

Link elements in shadow roots without rel=stylesheet are currently not
added as stylesheet candidates upon insertion. This causes a crash if
rel=stylesheet is set (and then loaded) later.

R=futhark@chromium.org

Bug: 886753
Change-Id: Ia0de2c1edf43407950f973982ee1c262a909d220
Reviewed-on: https://chromium-review.googlesource.com/1242463
Commit-Queue: Anders Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593907}
const QualifiedName& HTMLLinkElement::SubResourceAttributeName() const {
  if (DeprecatedEqualIgnoringCase(getAttribute(typeAttr), "text/css")) {
    return hrefAttr;
  }
  return HTMLElement::SubResourceAttributeName();
}

const QualifiedName& HTMLLinkElement::SubResourceAttributeName() const {
  if (DeprecatedEqualIgnoringCase(getAttribute(typeAttr), "text/css")) {
    return hrefAttr;
  }
  return HTMLElement::SubResourceAttributeName();
}
