cocoa: Fix for Mac bookmark drag regression.

BUG=307385
TEST=
1. Launch chrome & bookmark ntp or any webpage, save it in "Bookmarks Bar"
2. Navigate to chrome://bookmarks/, create a "New Folder" in "Bookmarks Bar"
3. Now drag the bookmarked page in "New Folder" & observe

After dragging, the bookmarked page should not be seen anymore as it's already
moved to "New Folder".
R=asvitkine@chromium.org

Review URL: https://codereview.chromium.org/27301004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228840 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228840