A style's scoping node should be a shadow host if the style is registered in a shadow root.

W3C spec bug:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=20957

Design Doc:
https://docs.google.com/document/d/12StL2pVsdkMj_gyJ6Oux2l8_nKakUkTHAqnYC4GIv8w/edit?usp=sharing

Example:
https://docs.google.com/document/d/1GWUtAjDUtJ8Ae0Yt3rBVsFmfpfoUa5ALK3WNWCHRtVk/edit?usp=sharing

TEST=fast/dom/shadow/shadow-tree-styles-select-host.html
BUG=247275
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/19500004

git-svn-id: svn://svn.chromium.org/blink/trunk@155737 bbb929c8-8fbe-4397-9dbb-9b2b20218538