Don't populate URL data in WebDropData when dragging files.

      This is considered a potential security issue as well, since it leaks
filesystem paths.

BUG=332579

Review URL: https://codereview.chromium.org/135633002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244538 0039d316-1c4b-4281-b951-d872f2087c98