For PyAuto automation on ChromeOS, add "stable-channel" to list of valid channels.
For PyAuto automation on ChromeOS, add "stable-channel" to list of valid channels.

BUG=None.
TEST=Manual.

Review URL: http://codereview.chromium.org/6901117

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@83564 0039d316-1c4b-4281-b951-d872f2087c98

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