Add a UI test for downloading in incognito mode.
Add a UI test for downloading in incognito mode.

This test opens an incognito window and verifies that
downloads work and that closing the incognito window
does not crash (see bug http://crbug.com/13983).

TEST=Test should run and not crash.
BUG=none

Review URL: http://codereview.chromium.org/126181

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@18534 0039d316-1c4b-4281-b951-d872f2087c98

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