[Presentation API] Implement ondefaultsessionstart in PSImpl.

content/renderer/presentation/presentation_dispatcher.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/renderer/presentation/presentation_dispatcher.h"
 
 #include "base/logging.h"
 #include "content/common/presentation/presentation_service.mojom.h"
 #include "content/public/common/service_registry.h"
 #include "content/public/renderer/render_frame.h"
@@ skipping 154 matching lines @@
 void PresentationDispatcher::OnDefaultSessionStarted(
     presentation::PresentationSessionInfoPtr session_info) {
   if (!controller_)
     return;
 
   // Reset the callback to get the next event.
   presentation_service_->ListenForDefaultSessionStart(base::Bind(
       &PresentationDispatcher::OnDefaultSessionStarted,
       base::Unretained(this)));
 
-  DCHECK(!session_info.is_null());
-  controller_->didStartDefaultSession(
-      new PresentationSessionClient(session_info.Pass()));
+  if (!session_info.is_null()) {
+    controller_->didStartDefaultSession(
+        new PresentationSessionClient(session_info.Pass()));
+  }
 }
 
 void PresentationDispatcher::OnSessionCreated(
     blink::WebPresentationSessionClientCallbacks* callback,
     presentation::PresentationSessionInfoPtr session_info,
     presentation::PresentationErrorPtr error) {
   DCHECK(callback);
   if (!error.is_null()) {
     DCHECK(session_info.is_null());
     callback->onError(new blink::WebPresentationError(
@@ skipping 34 matching lines @@
   presentation_service_->ListenForDefaultSessionStart(base::Bind(
       &PresentationDispatcher::OnDefaultSessionStarted,
       base::Unretained(this)));
   presentation_service_->ListenForSessionStateChange(base::Bind(
       &PresentationDispatcher::OnSessionStateChange,
       base::Unretained(this)));
   */
 }
 
 }  // namespace content