Remove histogram in ShadowRoot constructor
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@168882 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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