[Tools] Add scm-remove-wrong-checkout.py.

Let's start preparing the ground for v8-crosswalk.

`scm-remove-wrong-checkout.py` is a helper script intended to be invoked via
`gclient recurse`. `fetch_deps.py` calls it automatically.

It is intended to remove a directory pointed to by a gclient entry when it is
supposed to be a git checkout but contains an svn one (and vice-versa).

The main idea is to prepare the ground for using our v8-crosswalk repository:
since V8 so far has been cloned from Google's SVN repository, if we then switch
to our git repository gclient sync will fail.