Prevents down scrolls in mid-home-gesture event from being ignored

Bug: 896790
Change-Id: I8fef79fbbd3790d344ad92170f506fe46feefdd6
Reviewed-on: https://chromium-review.googlesource.com/c/1308600Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Kevin Strohbehn <ginko@google.com>
Cr-Commit-Position: refs/heads/master@{#604056}

ash/shelf/shelf_layout_manager.cc

@@ -1194,8 +1194,9 @@ bool ShelfLayoutManager::ShouldHomeGestureHandleEvent(float scroll_y) const {
   if (!IsVisible())
     return false;
 
-  // Scroll down events should never be handled.
-  if (scroll_y >= 0)
+  // Scroll down events should never be handled, unless they are currently being
+  // handled
+  if (scroll_y >= 0 && gesture_drag_status_ != GESTURE_DRAG_APPLIST_IN_PROGRESS)
     return false;
 
   return true;