Implements ListenForSessionMessages in PresentationServiceImpl; uses Swap to avoid copying large da…

content/common/presentation/presentation_service.mojom

Index: content/common/presentation/presentation_service.mojom
diff --git a/content/common/presentation/presentation_service.mojom b/content/common/presentation/presentation_service.mojom
index 1ec3d7cddfbfeaffac102cad6403d513daeec294..79d1d2071854bf2e8aad3b44c3e0ec34c985d11b 100644
--- a/content/common/presentation/presentation_service.mojom
+++ b/content/common/presentation/presentation_service.mojom
@@ -98,6 +98,8 @@ interface PresentationService {
             PresentationSessionState newState);
 
     // Called when the frame is ready to process the next batch of messages.
+    // When the callback carries null messages, there is an error
+    // at the presentation service side.
     ListenForSessionMessages()
-        => (array<SessionMessage> messages);
+        => (array<SessionMessage>? messages);
 };