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}
  explicit ScrollableShelfFocusSearch(
      ScrollableShelfView* scrollable_shelf_view)
      : FocusSearch(/*root=*/nullptr,
                    /*cycle=*/true,
                    /*accessibility_mode=*/true),
        scrollable_shelf_view_(scrollable_shelf_view) {}

  explicit ScrollableShelfFocusSearch(
      ScrollableShelfView* scrollable_shelf_view)
      : FocusSearch(/*root=*/nullptr,
                    /*cycle=*/true,
                    /*accessibility_mode=*/true),
        scrollable_shelf_view_(scrollable_shelf_view) {}
