Cros: Fix overlapped tooltip for shelf scrollable arrows

Sets the height of the arrow button to match the bounds of the other
shelf buttons to place the tooltip at the same height.

Bug: 1043009
Change-Id: I7c83a825c41a5378eefc520023dcc25e6e25de64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2033821Reviewed-by: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Ana Salazar <anasalazar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737919}

ash/shelf/scrollable_shelf_view.cc

@@ -813,8 +813,9 @@ gfx::Size ScrollableShelfView::CalculatePreferredSize() const {
 }
 
 void ScrollableShelfView::Layout() {
-  gfx::Size arrow_button_size(kArrowButtonSize, kArrowButtonSize);
   gfx::Rect shelf_container_bounds = gfx::Rect(size());
+  gfx::Size arrow_button_size(kArrowButtonSize,
+                              shelf_container_bounds.height());
 
   // Transpose and layout as if it is horizontal.
   const bool is_horizontal = GetShelf()->IsHorizontalAlignment();