[PresentationAPI] Added on-session-message handler.

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 91da501913f3637a7f6912d88b5151b2f72489eb..1ec3d7cddfbfeaffac102cad6403d513daeec294 100644
--- a/content/common/presentation/presentation_service.mojom
+++ b/content/common/presentation/presentation_service.mojom
@@ -26,6 +26,19 @@ struct PresentationError {
     string message;
 };
 
+enum PresentationMessageType {
+   TEXT,
+   ARRAY_BUFFER,
+};
+
+struct SessionMessage {
+   string presentation_url;
+   string presentation_id;
+   PresentationMessageType type;
+   string? message;
+   array<uint8>? data;
+};
+
 interface PresentationService {
     // Called when the frame sets or changes the default presentation URL or
     // presentation ID.
@@ -83,4 +96,8 @@ interface PresentationService {
     ListenForSessionStateChange()
         => (PresentationSessionInfo sessionInfo,
             PresentationSessionState newState);
+
+    // Called when the frame is ready to process the next batch of messages.
+    ListenForSessionMessages()
+        => (array<SessionMessage> messages);
 };