[ios] Fix recent tabs crash in iOS 10

Previously, model changes and table updates were not synchronized for
handling "Hide for now" on long presses in recent tabs.

This CL moves the model changes into the same block as table updates
so that they are fed together into |-performBatchUpdates:| in iOS 11
and |-beginUpdates| in iOS 10.

This was only crashing in iOS 10 iPad, but theoretically, it is wrong
and would crash in other configurations.

Bug: 851946
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I4fe35eb06cf77d1fb6097a5508d72dd1e93fc6ee
Reviewed-on: https://chromium-review.googlesource.com/1108346Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569175}

ios/chrome/browser/ui/recent_tabs/recent_tabs_table_view_controller.mm

@@ -1009,10 +1009,9 @@ const int kRecentlyClosedTabsSectionIndex = 0;
   sync_sessions::OpenTabsUIDelegate* openTabs =
       syncService->GetOpenTabsUIDelegate();
 
+  void (^tableUpdates)(void) = ^{
     [self.tableViewModel removeSectionWithIdentifier:sectionIdentifier];
     _syncedSessions->EraseSession(section - kNumberOfSectionsBeforeSessions);
-
-  void (^tableUpdates)(void) = ^{
     [self.tableView deleteSections:[NSIndexSet indexSetWithIndex:section]
                   withRowAnimation:UITableViewRowAnimationLeft];
   };