Add comment explaining when |display_private_| can be null

Based on discussions here[1], it isn't obvious why |display_private_|
should be null checked before use. Adding a comment here from this
discussion should make this clearer.

[1]: https://chromium-review.googlesource.com/c/chromium/src/+/2259015/21/ui/compositor/compositor.cc#774

Bug: 1107926
Change-Id: I5109c72bb6223a4b79ab1706a9dffe08d6a23299
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315091Reviewed-by: kylechar <kylechar@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Mario Bianucci <mabian@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#791402}

ui/compositor/compositor.h

@@ -416,6 +416,13 @@ class COMPOSITOR_EXPORT Compositor : public cc::LayerTreeHostClient,
 
   ui::ContextFactory* context_factory_;
 
+  // |display_private_| can be null for:
+  // 1. Tests that don't set |display_private_|.
+  // 2. Intermittently on creation or if there is some kind of error (GPU crash,
+  //    GL context loss, etc.) that triggers reinitializing message pipes to the
+  //    GPU process RootCompositorFrameSinkImpl.
+  // Therefore, it should always be null checked for safety before use.
+  //
   // These pointers are owned by |context_factory_|, and must be reset before
   // calling RemoveCompositor();
   viz::mojom::DisplayPrivate* display_private_ = nullptr;