Rename EventNames to ThreadLocalEventNames

This class is thread-local, and we're planning to use the name "EventNames" for
the new autogenerated event names.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/26341005

git-svn-id: svn://svn.chromium.org/blink/trunk@159070 bbb929c8-8fbe-4397-9dbb-9b2b20218538