Fix DnD jank

The mouseover code I added to do *smart* tooltips was making DnD almost
unusable.

BUG=None

TEST=Drag thumbnails on the NNTP and they should move smoothly.

Review URL: http://codereview.chromium.org/149427

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@20413 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/resources/new_new_tab.js

@@ -1055,6 +1055,12 @@ window.addEventListener('keydown', function(e) {
 
 // Tooltip for elements that have text that overflows.
 document.addEventListener('mouseover', function(e) {
+  // We don't want to do this while we are dragging because it makes things very
+  // janky
+  if (dnd.dragItem) {
+    return;
+  }
+
   var el = findAncestor(e.target, function(el) {
     return el.xtitle;
   });