Revert 169711 "Prevent web content from forging File entries in ..."

As it turns out, we only needed to patch the Chrome side.

> Prevent web content from forging File entries in drag and drop.
> 
> There are two separate bugs that this and the corresponding Chrome patch
> aim to address:
> - On Linux, files and URLs are transferred in the same MIME type, so
>   it's impossible to tell if a filename was set by a trusted source or
>   forged by web content.
> - DownloadURL triggers the download of potentially cross-origin content.
>   On some platforms, such as Windows, the resulting download is treated
>   as a file drag by Chrome, allowing web content to read cross origin
>   content.
> 
> In order to prevent web content from doing this, drags initiated by a
> renderer will be marked as tainted. When tainted drags are over web
> content, Blink will only allow the resulting filename to be used for
> navigation, with Chrome enforcing this with the sandbox policy.
> 
> Unfortunately, this does break some potentially interesting use cases
> like being able to drag an attachment from Gmail to a file input, but
> those will have to be separately addressed, if possible.
> 
> BUG=346135
> R=abarth@chromium.org, tony@chromium.org
> 
> Review URL: https://codereview.chromium.org/193803002

TBR=dcheng@chromium.org

Review URL: https://codereview.chromium.org/211853002

git-svn-id: svn://svn.chromium.org/blink/trunk@169979 bbb929c8-8fbe-4397-9dbb-9b2b20218538