Add CF_HDROP path for dragging on Win Aura

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
SVN changes up to revision 244355