Remove histogram in ShadowRoot constructor

This histogram takes too much time. If we want to track these stats, we can use
a UseCounter instead.

R=esprehn@chromium.org
BUG=351135

Review URL: https://codereview.chromium.org/193683002

git-svn-id: svn://svn.chromium.org/blink/trunk@168882 bbb929c8-8fbe-4397-9dbb-9b2b20218538