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}
int CalculateOverflowPadding(int available_size) {
  return (available_size - kArrowButtonGroupWidth -
          ShelfConfig::Get()->button_size() - GetAppIconEndPadding()) %
         GetUnit();
}

int CalculateOverflowPadding(int available_size) {
  return (available_size - kArrowButtonGroupWidth -
          ShelfConfig::Get()->button_size() - GetAppIconEndPadding()) %
         GetUnit();
}
