[ios] Fix rotations in tab grid

The tab grid was not behaving properly in rotations. The page would
change when it is not supposed to. This is due to the page being set
inside |-scrollViewDidScroll:| while the rotation was occurring.
The fix is to separately set the page for non-user-dragging scroll
in |-scrollViewDidEndScrollingAnimation:|.

Bug: 828735
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ib325eb6ef5d5ebb98274ec565cdbb03a21b8e9e1
Reviewed-on: https://chromium-review.googlesource.com/998055Reviewed-by: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548541}

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

@@ -29,6 +29,14 @@ typedef NS_ENUM(NSUInteger, TabGridConfiguration) {
   TabGridConfigurationBottomToolbar = 1,
   TabGridConfigurationFloatingButton,
 };
+
+// Computes the page from the offset and width of |scrollView|.
+TabGridPage GetPageFromScrollView(UIScrollView* scrollView) {
+  // TODO(crbug.com/822328) : Fix for RTL.
+  CGFloat pageWidth = scrollView.frame.size.width;
+  float fractionalPage = scrollView.contentOffset.x / pageWidth;
+  return static_cast<TabGridPage>(lround(fractionalPage));
+}
 }  // namespace
 
 @interface TabGridViewController ()<GridViewControllerDelegate,
@@ -169,19 +177,20 @@ typedef NS_ENUM(NSUInteger, TabGridConfiguration) {
         self.scrollView.contentSize.width - self.scrollView.frame.size.width;
     CGFloat offset = scrollView.contentOffset.x / offsetWidth;
     self.topToolbar.pageControl.sliderPosition = offset;
-  }
 
-  // Bookkeeping for the current page.
-  // TODO(crbug.com/822328) : Fix for RTL.
-  CGFloat pageWidth = scrollView.frame.size.width;
-  float fractionalPage = scrollView.contentOffset.x / pageWidth;
-  NSUInteger page = lround(fractionalPage);
-  if (page != self.currentPage) {
-    _currentPage = static_cast<TabGridPage>(page);
-    [self configureButtonsForOriginalAndCurrentPage];
+    TabGridPage page = GetPageFromScrollView(scrollView);
+    if (page != _currentPage) {
+      _currentPage = page;
+      [self configureButtonsForOriginalAndCurrentPage];
+    }
   }
 }
 
+- (void)scrollViewDidEndScrollingAnimation:(UIScrollView*)scrollView {
+  _currentPage = GetPageFromScrollView(scrollView);
+  [self configureButtonsForOriginalAndCurrentPage];
+}
+
 #pragma mark - UIScrollViewAccessibilityDelegate
 
 - (NSString*)accessibilityScrollStatusForScrollView:(UIScrollView*)scrollView {
@@ -280,7 +289,7 @@ typedef NS_ENUM(NSUInteger, TabGridConfiguration) {
     _currentPage = currentPage;
   } else {
     [self.scrollView setContentOffset:offset animated:YES];
-    // _currentPage is set in scrollViewDidScroll:
+    // _currentPage is set in scrollViewDidEndScrollingAnimation:
   }
 }