Make page switch on drag happen at top and bottom for experimental app list.

This CL makes dragging an app in the experimental app list change pages at
the top and bottom of the apps grid rather than at the sides. This matches
the scroll behavior of the experimental app list.

BUG=406222

Review URL: https://codereview.chromium.org/518673008

Cr-Commit-Position: refs/heads/master@{#293673}

ui/app_list/views/apps_grid_view.cc

@@ -1652,22 +1652,30 @@ void AppsGridView::MaybeStartPageFlipTimer(const gfx::Point& drag_point) {
     StopPageFlipTimer();
   int new_page_flip_target = -1;
 
-  if (page_switcher_view_ &&
-      page_switcher_view_->bounds().Contains(drag_point)) {
-    gfx::Point page_switcher_point(drag_point);
-    views::View::ConvertPointToTarget(this, page_switcher_view_,
-                                      &page_switcher_point);
-    new_page_flip_target =
-        page_switcher_view_->GetPageForPoint(page_switcher_point);
-  }
+  // Drag zones are at the edges of the scroll axis.
+  if (pagination_controller_->scroll_axis() ==
+      PaginationController::SCROLL_AXIS_VERTICAL) {
+    if (drag_point.y() < kPageFlipZoneSize)
+      new_page_flip_target = pagination_model_.selected_page() - 1;
+    else if (drag_point.y() > height() - kPageFlipZoneSize)
+      new_page_flip_target = pagination_model_.selected_page() + 1;
+  } else {
+    if (page_switcher_view_->bounds().Contains(drag_point)) {
+      gfx::Point page_switcher_point(drag_point);
+      views::View::ConvertPointToTarget(
+          this, page_switcher_view_, &page_switcher_point);
+      new_page_flip_target =
+          page_switcher_view_->GetPageForPoint(page_switcher_point);
+    }
 
-  // TODO(xiyuan): Fix this for RTL.
-  if (new_page_flip_target == -1 && drag_point.x() < kPageFlipZoneSize)
-    new_page_flip_target = pagination_model_.selected_page() - 1;
+    // TODO(xiyuan): Fix this for RTL.
+    if (new_page_flip_target == -1 && drag_point.x() < kPageFlipZoneSize)
+      new_page_flip_target = pagination_model_.selected_page() - 1;
 
-  if (new_page_flip_target == -1 &&
-      drag_point.x() > width() - kPageFlipZoneSize) {
-    new_page_flip_target = pagination_model_.selected_page() + 1;
+    if (new_page_flip_target == -1 &&
+        drag_point.x() > width() - kPageFlipZoneSize) {
+      new_page_flip_target = pagination_model_.selected_page() + 1;
+    }
   }
 
   if (new_page_flip_target == page_flip_target_)