[sync] WebContentsObserver based sessions notifications

ios/chrome/browser/sync/ios_chrome_synced_tab_delegate.mm

Index: ios/chrome/browser/sync/ios_chrome_synced_tab_delegate.mm
diff --git a/ios/chrome/browser/sync/ios_chrome_synced_tab_delegate.mm b/ios/chrome/browser/sync/ios_chrome_synced_tab_delegate.mm
index 67a293675af802864e754c0686c30e29c110def3..680dec4305d25e09fbb0e9f1e6459ce2dd6e82bb 100644
--- a/ios/chrome/browser/sync/ios_chrome_synced_tab_delegate.mm
+++ b/ios/chrome/browser/sync/ios_chrome_synced_tab_delegate.mm
@@ -52,6 +52,11 @@ bool IOSChromeSyncedTabDelegate::IsBeingDestroyed() const {
   return web_state_->IsBeingDestroyed();
 }
 
+// todo(pnoland): add logic to store and return the source tab id on ios.
+SessionID::id_type IOSChromeSyncedTabDelegate::GetSourceTabID() const {
+  return sync_sessions::kUnknownTabID;
+}
+
 std::string IOSChromeSyncedTabDelegate::GetExtensionAppId() const {
   return std::string();
 }