Add test coverage for the childrenSupportStyleSharing check

We didn't have any good test coverage for when the RestyleFlags
dynamically change after adding a candidate to the style sharing list,
so lets add some.

Review URL: https://codereview.chromium.org/262833003

git-svn-id: svn://svn.chromium.org/blink/trunk@173172 bbb929c8-8fbe-4397-9dbb-9b2b20218538