Adding a debug mode where we print style sharing statistics whenever we recalc
style. This will let us gather some hard numbers on the efficacy of our current
style sharing logic, and compare it to other solutions.

Review URL: https://chromiumcodereview.appspot.com/19699008

git-svn-id: svn://svn.chromium.org/blink/trunk@154601 bbb929c8-8fbe-4397-9dbb-9b2b20218538