Mark correct tree-scope dirty removing link in shadow.

Noticed while working on 567021, StyleElement found the correct
tree-scope in from of a shadow root when applicable while
HTMLLinkElement would just use the document. This is what kept us from
being able to use AnalyzedStyleUpdate for removedFrom() for link
elements.

R=meade@chromium.org
BUG=671596

Review-Url: https://codereview.chromium.org/2554873002
Cr-Commit-Position: refs/heads/master@{#437210}
6 files changed