Rename sheriffbot to webkitbot
https://bugs.webkit.org/show_bug.cgi?id=114109

Reviewed by Benjamin Poulain.

Renamed since sheriffbot will be used in Blink.

* Scripts/webkitpy/tool/commands/sheriffbot.py:
(SheriffBot):
(SheriffBot.begin_work_queue):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147865 268f45cc-cd09-0410-ab3c-d52691b4dbfc