Add UseCounter for initMessageEvent (including prefixed version.)

Compile some data on whether these init*Event() methods can indeed be
retired.

R=
BUG=311332

Review URL: https://codereview.chromium.org/112583005

git-svn-id: svn://svn.chromium.org/blink/trunk@164098 bbb929c8-8fbe-4397-9dbb-9b2b20218538