Refactor ScrollableShelf on |space_for_icons_|

|space_for_icons_| indicates the available space in scrollable shelf
to accommodate shelf icons. Now it is an integer type. Replace it with
gfx::Rect.

Bug: 997807
Change-Id: I4f9ba3206bd69dfdaf50894de46239e676db6454
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1801326
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696446}
void ScrollableShelfView::ButtonPressed(views::Button* sender,
                                        const ui::Event& event,
                                        views::InkDrop* ink_drop) {
  views::View* sender_view = sender;
  DCHECK((sender_view == left_arrow_) || (sender_view == right_arrow_));

  ScrollToNewPage(sender_view == right_arrow_);
}

void ScrollableShelfView::ButtonPressed(views::Button* sender,
                                        const ui::Event& event,
                                        views::InkDrop* ink_drop) {
  views::View* sender_view = sender;
  DCHECK((sender_view == left_arrow_) || (sender_view == right_arrow_));

  ScrollToNewPage(sender_view == right_arrow_);
}
