[ios] Synchronize tab grid page control with scroll view

Previously there was a bug where there was a mismatch of internal states
between the page control and the scroll view.
It is important that they both are synchronized with the same current
page.
This CL fixes the lack of synchronization.

TBR=marq@chromium.org
Bug: 825898

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I284460d9581a05f780057ef53715484254978859
Reviewed-on: https://chromium-review.googlesource.com/981302
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545953}

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

@@ -195,6 +195,14 @@ NSString* StringForItemCount(long count) {
   self.selectedImageView.center =
       [self convertPoint:RectCenter(self.bounds) toView:self.sliderView];
   _sliderPosition = sliderPosition;
+
+  // |_selectedPage| should be kept in sync with the slider position.
+  if (sliderPosition < 0.25)
+    _selectedPage = TabGridPageIncognitoTabs;
+  else if (sliderPosition < 0.75)
+    _selectedPage = TabGridPageRegularTabs;
+  else
+    _selectedPage = TabGridPageRemoteTabs;
 }
 
 // Setters for the control's text values. These need to update three things:

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

@@ -164,20 +164,21 @@ typedef NS_ENUM(NSUInteger, TabGridConfiguration) {
 
 - (void)scrollViewDidScroll:(UIScrollView*)scrollView {
   if (scrollView.dragging || scrollView.decelerating) {
+    // Only when user initiates scroll through dragging.
     CGFloat offsetWidth =
         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];
-    }
+  // 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];
   }
 }
 
@@ -279,7 +280,7 @@ typedef NS_ENUM(NSUInteger, TabGridConfiguration) {
     _currentPage = currentPage;
   } else {
     [self.scrollView setContentOffset:offset animated:YES];
-    // _currentPage is set in scrollViewDidEndDecelerating:
+    // _currentPage is set in scrollViewDidScroll:
   }
 }