Add a unit test that filenames aren't unintentionally converted to URLs.

Also fixes two issues in OSExchangeDataProviderWin:
- It used a disjoint set of clipboard formats when handling
  GetUrl(..., true /* filename conversion */) vs GetFilenames(...), so the
  actual returned results would vary depending on which one was called.
- It incorrectly used ::DragFinish() instead of ::ReleaseStgMedium().
  ::DragFinish() is only meant to be used in conjunction with WM_DROPFILES.

BUG=346135

Review URL: https://codereview.chromium.org/380553002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283226 0039d316-1c4b-4281-b951-d872f2087c98
  BOOL Empty() {
    if (!opened_) {
      NOTREACHED();
      return false;
    }
    return ::EmptyClipboard();
  }

  BOOL Empty() {
    if (!opened_) {
      NOTREACHED();
      return false;
    }
    return ::EmptyClipboard();
  }
