Add a test case that shows the amount of recalc we do we adding/removing stylesheets.

This is in preparation for reworking StyleSheetCollection::compareStyleSheets.

Review URL: https://codereview.chromium.org/82003002

git-svn-id: svn://svn.chromium.org/blink/trunk@162507 bbb929c8-8fbe-4397-9dbb-9b2b20218538