[ios] Move RemoteTabs additionalSafeAreaInset modification in Tabgrid to during rotation animation

Currently making modifications in the completion block results in flickers during rotations.

Video: https://drive.google.com/open?id=1rtjOugnQaVRqO9mtflrEIgUJ16oipic2

Bug: 878832
Change-Id: Icee69d81199ff068c3ac74a768d0ee4509372826
Reviewed-on: https://chromium-review.googlesource.com/1199931Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588521}

ios/chrome/browser/ui/tab_grid/tab_grid_view_controller.mm

@@ -251,13 +251,9 @@ NSUInteger GetPageIndexFromPage(TabGridPage page) {
     // current page value.
     self.currentPage = _currentPage;
     [self configureViewControllerForCurrentSizeClassesAndPage];
+    [self setInsetForRemoteTabs];
   };
-  auto completion =
-      ^(id<UIViewControllerTransitionCoordinatorContext> context) {
-        // Modify Remote Tabs Insets when page appears and during rotation.
-        [self setInsetForRemoteTabs];
-      };
-  [coordinator animateAlongsideTransition:animate completion:completion];
+  [coordinator animateAlongsideTransition:animate completion:nil];
 }
 
 - (UIStatusBarStyle)preferredStatusBarStyle {