Update gpu_surface_metal_impeller.mm

shell/gpu/gpu_surface_metal_impeller.mm

@@ -150,6 +150,8 @@
         auto surface = impeller::SurfaceMTL::MakeFromMetalLayerDrawable(renderer->GetContext(),
                                                                         drawable, clip_rect);
 
+        // The surface may be null if we failed to allocate the onscreen render target
+        // due to running out of memory.
         if (!surface) {
           return false;
         }