[pyauto] Use windex=None to signify automation commands that do not target a particular window
[pyauto] Use windex=None to signify automation commands that do not target a particular window

Use windex=None, instead of windex=-1.
The former seems more intuitive. However, all uses of windex=-1 should
continue to work as is, for backwards compatibility.

BUG=None
TEST=pyauto tests green

R=dennisjeffrey@chromium.org

Review URL: https://chromiumcodereview.appspot.com/9233052

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@119923 0039d316-1c4b-4281-b951-d872f2087c98

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