Cros: Prevent hotseat from losing padding when dragged

Just add the padding when the drag starts.

Bug: 1009120
Change-Id: Ia7e2216b3030b6bf2835a591a5fc1312b3eb9a65
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1829554
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Auto-Submit: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700928}

ash/shelf/shelf_layout_manager.cc

@@ -2039,7 +2039,8 @@ bool ShelfLayoutManager::StartShelfDrag(
   // offset to the hotseats extended position.
   if (state_.hotseat_state == HotseatState::kExtended &&
       visibility_state() == SHELF_VISIBLE) {
-    drag_amount_ = -ShelfConfig::Get()->hotseat_size();
+    drag_amount_ = -(ShelfConfig::Get()->hotseat_size() +
+                     ShelfConfig::Get()->hotseat_bottom_padding());
   } else {
     drag_amount_ = 0.f;
   }