Add comment to Widget::GetCompositor() to clarify association lifetime

Bug: 1014863
Change-Id: I6896cee115e56d33a691c7ac796c42b77efe0afa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1864506
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706558}

ui/views/widget/widget.h

@@ -769,6 +769,8 @@ class VIEWS_EXPORT Widget : public internal::NativeWidgetDelegate,
     return non_client_view_ ? non_client_view_->client_view() : nullptr;
   }
 
+  // Returns the compositor for this Widget, note that this may change during
+  // the Widget's lifetime (e.g. when switching monitors on Chrome OS).
   ui::Compositor* GetCompositor() {
     return const_cast<ui::Compositor*>(
         const_cast<const Widget*>(this)->GetCompositor());