Allow :host()::before and :host::after to work as expected

In style definition in a shadow tree, :host()::before and
:host()::after should match and work as expected.
(i.e. prepend/append "content" to its shadow host).

BUG=393509
TEST=pass layout tests

Review URL: https://codereview.chromium.org/402813003

git-svn-id: svn://svn.chromium.org/blink/trunk@180888 bbb929c8-8fbe-4397-9dbb-9b2b20218538