Fix the broken page flip zone in app list

Background:
Recently, apps grid view's bounds changed. Also the shelf can be set
auto-hidden so that the display work area is always full screen even
when the shelf is visible. Thus, incorrect bounds are checked for
page flip during dragging.

Change:
Change the page flip zone to make the page flip only happen when drag
point is between the bottom of apps grid view and the top of bottom
shelf.

BUG=840021

Change-Id: Ia7f1be3ff6a2ae04bc41d43391ab9f525efd1b90
Reviewed-on: https://chromium-review.googlesource.com/1045611
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556485}

ui/app_list/views/app_list_view.h

@@ -207,6 +207,8 @@ class APP_LIST_EXPORT AppListView : public views::WidgetDelegateView,
 
   bool is_tablet_mode() const { return is_tablet_mode_; }
 
+  bool is_side_shelf() const { return is_side_shelf_; }
+
   bool is_in_drag() const { return is_in_drag_; }
 
   int app_list_y_position_in_screen() const {

ui/app_list/views/apps_grid_view.cc

@@ -2204,9 +2204,14 @@ bool AppsGridView::IsPointWithinBottomDragBuffer(
       display::Screen::GetScreen()->GetDisplayNearestView(
           GetWidget()->GetNativeView());
 
-  return point_in_screen.y() >
-             GetBoundsInScreen().height() - kPageFlipZoneSize &&
-         point_in_screen.y() < display.work_area().height();
+  const int kBottomDragBufferMin =
+      GetBoundsInScreen().bottom() - kPageFlipZoneSize;
+  const int kBottomDragBufferMax =
+      display.bounds().bottom() -
+      (contents_view_->app_list_view()->is_side_shelf() ? 0 : kShelfSize);
+
+  return point_in_screen.y() > kBottomDragBufferMin &&
+         point_in_screen.y() < kBottomDragBufferMax;
 }
 
 void AppsGridView::ButtonPressed(views::Button* sender,