Drag/drop of a file into a WebView on Windows needs to allow access to that file

Drag/drop of a file into a WebView on Windows needs to allow access to that file
https://bugs.webkit.org/show_bug.cgi?id=76753
<rdar://problem/10731719>

Reviewed by Alexey Proskuryakov.

The Windows drag/drop code path was missing the code to open a file read exception.

Added code to match the Mac version, while allowing directories and multiple files to
also be dragged (matching our previous behavior).

* UIProcess/win/WebView.cpp:
(WebKit::maybeCreateSandboxExtensionFromDragData): Added.
(WebKit::WebView::Drop): Add a universal read exception if we're dragging a file into a WebView to open it.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105558 268f45cc-cd09-0410-ab3c-d52691b4dbfc