Annotate intentional leak of UA style sheets.

BUG=chromium:121729,chromium:369404
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/289973009

git-svn-id: svn://svn.chromium.org/blink/trunk@174264 bbb929c8-8fbe-4397-9dbb-9b2b20218538