Rename EventNames to EventTypeNames

This CL prepares use to move the list of Event IDL interfaces from being
thread-local to being thread-safe. They'll use the "EventNames" name to be
consistent with EventTargetNames. Naming is hard. :(

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/26745007

git-svn-id: svn://svn.chromium.org/blink/trunk@159349 bbb929c8-8fbe-4397-9dbb-9b2b20218538