Move isTouchEventType out of EventNames

This CL moves isTouchEventType out of the EventNames class into a free
function. This transformation will be useful when we make the event names
static instead of thread-local.

I've also taken the opportunity to delete two unused functions in EventNames
and to add a missing const qualifier that will cause a compile failure when we
move to static event names.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/26414002

git-svn-id: svn://svn.chromium.org/blink/trunk@159069 bbb929c8-8fbe-4397-9dbb-9b2b20218538