[ui] Add explanatory comment on WidgetObserver usage.

Bug: 1055483
Change-Id: Ic4a205e0c801bbd1ccc4e4cbf80cfd7aff099f95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2087616
Auto-Submit: Wez <wez@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749158}

ui/views/accessibility/view_ax_platform_node_delegate_auralinux.cc

@@ -104,6 +104,8 @@ class AuraLinuxApplication : public ui::AXPlatformNodeDelegateBase,
 
   void OnWidgetDestroying(Widget* widget) override {
     auto iter = std::find(widgets_.begin(), widgets_.end(), widget);
+    // Since |widget| is about to be destroyed, there is no point in removing
+    // |this| from its list of observers.
     if (iter != widgets_.end())
       widgets_.erase(iter);
   }