CrOs Status Area: Use a bit mask to keep track of child visibility

The current code makes the assumption that only one child changes
visibility at a time. While this is generally true, there is no actual
guarantee for it. This change keeps track of what children are visible
individually.

Change-Id: Ibc51535c6e0e7e6ab7e167999800138ebc67471b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2131038
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755238}

ash/system/status_area_widget.cc

@@ -424,12 +424,14 @@ void StatusAreaWidget::AddTrayButton(TrayBackgroundView* tray_button) {
 }
 
 StatusAreaWidget::LayoutInputs StatusAreaWidget::GetLayoutInputs() const {
-  unsigned int number_of_visible_children = 0;
-  for (auto* child : tray_buttons_)
-    number_of_visible_children += child->GetVisible() ? 1 : 0;
+  long child_visibility_bitmask = 0;
+  DCHECK(tray_buttons_.size() <
+         std::numeric_limits<decltype(child_visibility_bitmask)>::digits);
+  for (unsigned int i = 0; i < tray_buttons_.size(); ++i)
+    child_visibility_bitmask |= (tray_buttons_[i]->GetVisible() ? 1 : 0) << i;
   return {target_bounds_, CalculateCollapseState(),
           shelf_->shelf_layout_manager()->GetOpacity(),
-          number_of_visible_children};
+          child_visibility_bitmask};
 }
 
 }  // namespace ash

ash/system/status_area_widget.h

@@ -137,18 +137,13 @@ class ASH_EXPORT StatusAreaWidget : public SessionObserver,
     gfx::Rect bounds;
     CollapseState collapse_state = CollapseState::NOT_COLLAPSIBLE;
     float opacity = 0.0f;
-    // Children change visibility only one at a time, so keeping track of
-    // how many are visible (as opposed to the visibility state of each) is
-    // sufficient to make sure we don't miss necessary layout changes.
-    // TODO(manucornet): The assumption that children only change visibility
-    // one at a time may be too optimistic. One way to address this would be
-    // to have a child visibility change counter.
-    unsigned int number_of_visible_children = 0;
+    // Each bit keep track of one child's visibility.
+    long child_visibility_bitmask = 0;
 
     bool operator==(const LayoutInputs& other) const {
       return bounds == other.bounds && collapse_state == other.collapse_state &&
              opacity == other.opacity &&
-             number_of_visible_children == other.number_of_visible_children;
+             child_visibility_bitmask == other.child_visibility_bitmask;
     }
   };