Don't populate WebDropData with file URLs when dragging files.

      This is the Windows patch. There will be separate patches for Mac and Linux.

BUG=42685
TEST=none

Review URL: http://codereview.chromium.org/2126010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@47870 0039d316-1c4b-4281-b951-d872f2087c98