Add a regression to catch :host.foo should not match.

This came up during the code review at
https://codereview.chromium.org/402813003/

A CSS selector ":host.foo {}" in a shadow tree should never match
its shadow host, whereas ":host(.foo) {}" should match, of course.

BUG=393509
TEST=pass this test.

Review URL: https://codereview.chromium.org/482383004

git-svn-id: svn://svn.chromium.org/blink/trunk@180839 bbb929c8-8fbe-4397-9dbb-9b2b20218538