Disable the selection of promo cell in the new iOS bookmark ui

This CL prevents the selection of promo cell (the following screenshot)
https://drive.google.com/file/d/0B1dtd3IUt4I7RUh5MjdLRWtaYmM
by setting promo cell’s selection style to none.

Bug: 695749
Change-Id: I5df7ec5033a379f68970fa8cf11789b2a11078a9
Reviewed-on: https://chromium-review.googlesource.com/684634Reviewed-by: Eric Noyau <noyau@chromium.org>
Commit-Queue: Eric Noyau <noyau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504368}

ios/chrome/browser/ui/bookmarks/bookmark_table_view.mm

@@ -351,6 +351,7 @@ using IntegerPair = std::pair<NSInteger, NSInteger>;
     signinPromoCell.closeButtonAction = ^() {
       [weakSelf signinPromoCloseButtonAction];
     };
+    signinPromoCell.selectionStyle = UITableViewCellSelectionStyleNone;
     return signinPromoCell;
   }