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This ports the PrepareDragForDownload path over from the native windows
code path. This makes dragging files to explorer from, for example, gmail
save the content of the file, rather than a link, or the thumbnail
image.

(This needs to be merged to 32, so attempted to keep it semi-localized.)

BUG=332579,333320
R=ben@chromium.org

Review URL: https://codereview.chromium.org/135393002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244355 0039d316-1c4b-4281-b951-d872f2087c98