[Media Router] Do not issue a tab mirroring sinks query if tab is not valid.

chrome/browser/ui/webui/media_router/media_router_ui.cc

Index: chrome/browser/ui/webui/media_router/media_router_ui.cc
diff --git a/chrome/browser/ui/webui/media_router/media_router_ui.cc b/chrome/browser/ui/webui/media_router/media_router_ui.cc
index 52f2ec2cdea423065ec9f01448e1e1d72d363bc5..9b7aa31f98d291bbdcad6d1a8c7d671ddfc370c4 100644
--- a/chrome/browser/ui/webui/media_router/media_router_ui.cc
+++ b/chrome/browser/ui/webui/media_router/media_router_ui.cc
@@ -264,14 +264,16 @@ void MediaRouterUI::InitCommon(content::WebContents* initiator) {
   query_result_manager_.reset(new QueryResultManager(router_));
   query_result_manager_->AddObserver(this);
 
-  // These modes are always available.
+  // Desktop mirror mode is always available.
   query_result_manager_->StartSinksQuery(MediaCastMode::DESKTOP_MIRROR,
                                          MediaSourceForDesktop());
   initiator_ = initiator;
-  MediaSource mirroring_source(
-      MediaSourceForTab(SessionTabHelper::IdForTab(initiator)));
-  query_result_manager_->StartSinksQuery(MediaCastMode::TAB_MIRROR,
-                                         mirroring_source);
+  SessionID::id_type tab_id = SessionTabHelper::IdForTab(initiator);
+  if (tab_id != -1) {
+    MediaSource mirroring_source(MediaSourceForTab(tab_id));
+    query_result_manager_->StartSinksQuery(MediaCastMode::TAB_MIRROR,
+                                           mirroring_source);
+  }
   UpdateCastModes();
 }