[sync] WebContentsObserver based sessions notifications

chrome/browser/ui/sync/tab_contents_synced_tab_delegate.cc

Index: chrome/browser/ui/sync/tab_contents_synced_tab_delegate.cc
diff --git a/chrome/browser/ui/sync/tab_contents_synced_tab_delegate.cc b/chrome/browser/ui/sync/tab_contents_synced_tab_delegate.cc
index 636367a94ad33f8977b5815cf8f998d084d47bd5..eaf8ccc51e25315894eed3034c2dfc90871678b3 100644
--- a/chrome/browser/ui/sync/tab_contents_synced_tab_delegate.cc
+++ b/chrome/browser/ui/sync/tab_contents_synced_tab_delegate.cc
@@ -7,6 +7,7 @@
 #include "base/memory/ref_counted.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/sessions/session_tab_helper.h"
+#include "chrome/browser/sync/sessions/sync_sessions_router_tab_helper.h"
 #include "chrome/common/features.h"
 #include "components/sessions/content/content_serialized_navigation_builder.h"
 #include "components/sync_sessions/sync_sessions_client.h"
@@ -164,3 +165,10 @@ bool TabContentsSyncedTabDelegate::ShouldSync(
   }
   return false;
 }
+
+SessionID::id_type TabContentsSyncedTabDelegate::GetSourceTabID() const {
+  sync_sessions::SyncSessionsRouterTabHelper* helper =
+      sync_sessions::SyncSessionsRouterTabHelper::FromWebContents(
+          web_contents_);
+  return helper->source_tab_id();
+}