Generate thread-safe EventNames

In the next CL, we'll replace our use of thread-local event names with these
thread-safe event names.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/26340009

git-svn-id: svn://svn.chromium.org/blink/trunk@159139 bbb929c8-8fbe-4397-9dbb-9b2b20218538