My idea is to add a checkbox to the ForceScheduler. If the user
set checked it, CheckoutSource buildstep switched to "full" mode
with "fresh" method for this build. It means that buildslave
will revert/remove all files reported by svn status --no-ignore.
One potential challenge is that an svn checkout of WebKit can take a long time, sometimes so long that the checkout may time out such that you must run "svn cleanup", then try to check out a second time. (One way to mitigate this is to use a git repository from git.webkit.org, which doesn't have the timeout issue and would be faster when checking out from scratch.)