[Scrollable Shelf] Fix crash under extremely small display

As reported by Clusterfuzz, hotseat widget may not have enough space to
fully show even one shelf icon. Under that edge case, the end of the
last shelf icon will be outside the scrollable shelf. Based on the
current code, |last_visible_view_index| will be -1. This CL fixes the
issue.

Bug: 1094363
Change-Id: Ib4a49267c82a54e3c66f88df9702dddb216eca89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2250706
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779561}

ash/shelf/scrollable_shelf_view.cc

@@ -1965,8 +1965,13 @@ std::pair<int, int> ScrollableShelfView::CalculateTappableIconIndices(
     const int end_of_last_visible_view =
         last_visible_view_index * space_needed_for_button +
         shelf_view_->GetButtonSize() - scroll_distance_on_main_axis;
-    if (end_of_last_visible_view > visible_size)
+
+    // It is very rare but |visible_size| may be smaller than
+    // |space_needed_for_button| as reported in https://crbug.com/1094363.
+    if (end_of_last_visible_view > visible_size &&
+        last_visible_view_index > first_visible_view_index) {
       last_visible_view_index--;
+    }
   } else {
     DCHECK_EQ(layout_strategy, kShowLeftArrowButton);
     last_visible_view_index = visible_views_indices.size() - 1;