Adds a layout test that style in sibling shadow shouldn't match.

This test is based on sorvell@'s test case reported
at http://jsbin.com/ganocegu/4/edit for crbug.com/355674.

As the result of moving style in shadow's scope from
its host to its root, this issue has been fixed since
r176304.  This is the test case to catch any regression
about shadow's sibling matching.

BUG=355674
TEST=this layout test passes.

Review URL: https://codereview.chromium.org/330193006

git-svn-id: svn://svn.chromium.org/blink/trunk@176382 bbb929c8-8fbe-4397-9dbb-9b2b20218538