[ios] Update fullscreen insets during rotation animation

This change move the fullscreen inset updates to happen alongside the
size transition animation. The change in web content size will then
not happen after the transition finished, producing a cleaner experience.

Fixed: 1168154
Change-Id: I048c763ffbc0d3723833a003b3a9a79c8a8c58cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2641115Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845426}

ios/chrome/browser/ui/browser_view/browser_view_controller.mm

@@ -1686,13 +1686,16 @@ NSString* const kBrowserViewControllerSnackbarCategory =
         // Force updates of the toolbar updater as the toolbar height might
         // change on rotation.
         [_toolbarUIUpdater updateState];
-      }
-      completion:^(id<UIViewControllerTransitionCoordinatorContext> context) {
-        BrowserViewController* strongSelf = weakSelf;
         // Resize horizontal viewport if Smooth Scrolling is on.
         if (fullscreen::features::ShouldUseSmoothScrolling()) {
-          strongSelf.fullscreenController->ResizeHorizontalViewport();
+          BrowserViewController* strongSelf = weakSelf;
+          if (strongSelf) {
+            strongSelf.fullscreenController->ResizeHorizontalViewport();
+          }
         }
+      }
+      completion:^(id<UIViewControllerTransitionCoordinatorContext> context) {
+        BrowserViewController* strongSelf = weakSelf;
         if (!base::FeatureList::IsEnabled(kModernTabStrip)) {
           if (strongSelf.tabStripView) {
             [strongSelf.legacyTabStripCoordinator tabStripSizeDidChange];