Move mutation event histogramming over to UseCounter.

This gets rid of the one-off code and also changes it so we count
once per Page instead of once per Document.

Review URL: https://chromiumcodereview.appspot.com/24135002

git-svn-id: svn://svn.chromium.org/blink/trunk@157738 bbb929c8-8fbe-4397-9dbb-9b2b20218538