Add ...Last enum constant to blink types used in chrome IPC messages.

This is a pre-requisite to getting these values sanity checked by
chrome IPC.  We need to have these LAST constants defined so that
it is obvious that the they must be updated as part of any changes
that touch the enum.

R=abarth@chromium.org
BUG=246708

Review URL: https://codereview.chromium.org/143163004

git-svn-id: svn://svn.chromium.org/blink/trunk@166058 bbb929c8-8fbe-4397-9dbb-9b2b20218538