vim: Let ninja-build.vim detect Release correctly if out/Debug/protoc doesn't exist at all. BUG=none TEST=none Review URL: http://codereview.chromium.org/9265016 Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: 5956782071eb323f726315e166f2d84a5d135cc7