Use TreeScope::completeURL and baseURL instead of the Document versions

No change in behavior, as ShadowRoot::baseURL still delegates to its Document.

BUG=311682
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/126443005

git-svn-id: svn://svn.chromium.org/blink/trunk@165098 bbb929c8-8fbe-4397-9dbb-9b2b20218538