Remove unused ShadowRootUsageOriginType enum

Its usage was removed in https://src.chromium.org/viewvc/blink?view=rev&revision=168882

R=eseidel@chromium.org, abarth@chromium.org
BUG=351135

Review URL: https://codereview.chromium.org/196023003

git-svn-id: svn://svn.chromium.org/blink/trunk@168966 bbb929c8-8fbe-4397-9dbb-9b2b20218538