Add test for style recalcs when modifying stylesheets.

This shows that modifying via textContent/innerHTML correctly does
StyleInvalidationAnalysis and modifying via CSSOM does not.

Review URL: https://codereview.chromium.org/92693004

git-svn-id: svn://svn.chromium.org/blink/trunk@162800 bbb929c8-8fbe-4397-9dbb-9b2b20218538