Adds a dummy file so git users can run layout tests on Vista

Currently folks who use git to check out the chromium repo cannot run the layout tests on vista because run_webkit_tests.py checks for the existence of platform/chromium-win-vista and git, unlike SVN, will not create an empty directory if one exists upstream.  This works around the issue by adding a dummy file to platform/chromium-win-vista so the directory always is there.

Review URL: http://codereview.chromium.org/201128

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@26293 0039d316-1c4b-4281-b951-d872f2087c98

webkit/data/layout_tests/platform/chromium-win-vista/README.txt

+This directory is intentionally empty since all vista test expectations are in chromium-win.  However, if the directory is actually empty then folks using git to track the Chromium repo will not get a directory at all which makes run_webkit_tests.py a sad panda.  Thus this README is here to make sure the directory is added so tests can run.
+
+This file should be removed whenever vista-specific test expectations are checked into this directory.