[Mac] Ensure dragged button has a bookmark node when canceling bookmark bar drop

Bug: 840456
Change-Id: Idf022ee3493207ab64dd72d25a70f2527f557fad
Reviewed-on: https://chromium-review.googlesource.com/1048068Reviewed-by: Sidney San Martín <sdy@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557564}

chrome/browser/ui/cocoa/bookmarks/bookmark_bar_controller.mm

@@ -1999,7 +1999,7 @@ static BOOL ValueInRangeInclusive(CGFloat low, CGFloat value, CGFloat high) {
   }
   hasInsertionPos_ = NO;
   BookmarkButton* draggedButton = [BookmarkButton draggedButton];
-  if (!draggedButton) {
+  if (!draggedButton || [draggedButton bookmarkNode] == nullptr) {
     [self applyLayout:layout_ animated:YES];
     return;
   }