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::OnShelfAlignmentChanged(aura::Window* root_window) {
  const bool is_horizontal_alignment = GetShelf()->IsHorizontalAlignment();
  left_arrow_->set_is_horizontal_alignment(is_horizontal_alignment);
  right_arrow_->set_is_horizontal_alignment(is_horizontal_alignment);
  scroll_offset_ = gfx::Vector2dF();
  Layout();
}

void ScrollableShelfView::OnShelfAlignmentChanged(aura::Window* root_window) {
  const bool is_horizontal_alignment = GetShelf()->IsHorizontalAlignment();
  left_arrow_->set_is_horizontal_alignment(is_horizontal_alignment);
  right_arrow_->set_is_horizontal_alignment(is_horizontal_alignment);
  scroll_offset_ = gfx::Vector2dF();
  Layout();
}
