[sync] WebContentsObserver based sessions notifications

components/sync_sessions/synced_tab_delegate.h

Index: components/sync_sessions/synced_tab_delegate.h
diff --git a/components/sync_sessions/synced_tab_delegate.h b/components/sync_sessions/synced_tab_delegate.h
index 9781578c777463458dd41ee7b2c02bb0a5801f31..c69de8755d8d678661e4dd0bc187d807127c8d26 100644
--- a/components/sync_sessions/synced_tab_delegate.h
+++ b/components/sync_sessions/synced_tab_delegate.h
@@ -20,6 +20,8 @@ class SyncSessionsClient;
 
 namespace sync_sessions {
 
+enum InvalidTab { kInvalidTabID = -1 };
+
 // A SyncedTabDelegate is used to insulate the sync code from depending
 // directly on WebContents, NavigationController, and the extensions TabHelper.
 class SyncedTabDelegate {
@@ -31,6 +33,10 @@ class SyncedTabDelegate {
   virtual SessionID::id_type GetSessionId() const = 0;
   virtual bool IsBeingDestroyed() const = 0;
 
+  // Get the tab id of the tab responsible for opening this tab, if applicable.
+  // Returns kUnknownTabID(-1) if no such tab relationship is known.
+  virtual SessionID::id_type GetSourceTabID() const = 0;
+
   // Method derived from extensions TabHelper.
   virtual std::string GetExtensionAppId() const = 0;