[ios] [recent_tabs] Set imageDataSource before mediator starts observing

Currently, the image data source must be set before the mediator starts updating
its consumer. The mediator will start updating its consumer as soon as it
starts observing sync state, which starts before the view controller has been
presented. Therefore, the imageDataSource will DCHECK since it is being
accessed before it is set.

The order of setting the imageDataSource should not matter. So a bug
was filed for this: crbug.com/845636.

Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I9893493c1f0e2927b9bc6b69bfdea0142f03adab
Reviewed-on: https://chromium-review.googlesource.com/1069590Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560812}

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

@@ -66,11 +66,13 @@
   // Set the consumer first before calling [self.mediator initObservers] and
   // then [self.mediator reloadSessions].
   self.mediator.consumer = recentTabsTableViewController;
+  // TODO(crbug.com/845636) : Currently, the image data source must be set
+  // before the mediator starts updating its consumer. Fix this so that order of
+  // calls does not matter.
+  recentTabsTableViewController.imageDataSource = self.mediator;
   recentTabsTableViewController.delegate = self.mediator;
   [self.mediator initObservers];
   [self.mediator reloadSessions];
-  // Set the datasource for favicons.
-  recentTabsTableViewController.imageDataSource = self.mediator;
 
   // Present RecentTabsNavigationController.
   self.recentTabsNavigationController = [[TableViewNavigationController alloc]