[ios] Sets browserState before initingObservers on RecentTabs

Since [mediator initObservers] users the Browser state we need to set it before calling this method.

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I757b5c28f48c782b5984212ad39a7f27e643d21f
Reviewed-on: https://chromium-review.googlesource.com/994137Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547899}

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

@@ -65,9 +65,9 @@
   // longer than its associated VC.
   if (!self.mediator) {
     self.mediator = [[RecentTabsMediator alloc] init];
+    self.mediator.browserState = self.browserState;
     [self.mediator initObservers];
   }
-  self.mediator.browserState = self.browserState;
   self.mediator.consumer = recentTabsTableViewController;
   [self.mediator reloadSessions];