[iOS] Fix scroll view jumping in tab grid.

A previous CL arranged for the current panel of the tab grid to update
whenever the view layout was updated; this happens frequently when
scrolling, which results in a "jump" as soon as the scroll view passes
the threshold for changing panels. This CL fixes that.

Bug: 852247
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I7b2e56fdafff5cf156475563f04c5db0fd8964e6
Reviewed-on: https://chromium-review.googlesource.com/1099378Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566902}

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

@@ -149,8 +149,10 @@ NSUInteger GetPageIndexFromPage(TabGridPage page) {
 - (void)viewWillLayoutSubviews {
   [super viewWillLayoutSubviews];
   // Call the current page setter to sync the scroll view offset to the current
-  // page value. Don't animate this.
-  self.currentPage = _currentPage;
+  // page value, if the scroll view isn't scrolling. Don't animate this.
+  if (!self.scrollView.dragging && !self.scrollView.decelerating) {
+    self.currentPage = _currentPage;
+  }
   // The content inset of the tab grids must be modified so that the toolbars
   // do not obscure the tabs. This may change depending on orientation.
   UIEdgeInsets contentInset = UIEdgeInsetsZero;