Cancel a DownloadItem when the file selection dialog is cancelled.
Cancel a DownloadItem when the file selection dialog is cancelled.

BUG=114196
TEST=see bug for manual test


Review URL: http://codereview.chromium.org/9402011

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@123028 0039d316-1c4b-4281-b951-d872f2087c98

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