Keep an alias to RemoveDirectory, will be removed in a later change.

TEST=new unit test to test this function
BUG=none

Review URL: http://codereview.chromium.org/6628032

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@77463 0039d316-1c4b-4281-b951-d872f2087c98