[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 18e5a23c9b6166c8f5b616a36e6d79b80188d71b..8e7f12ffdb07a1e3b4c2c75acce667845967228f 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 +52,12 @@ bool IOSChromeSyncedTabDelegate::IsBeingDestroyed() const {
   return web_state_->IsBeingDestroyed();
 }
 
+// todo(pnoland): add logic to store and return the source tab id on ios.
+// http://crbug/695241
+SessionID::id_type IOSChromeSyncedTabDelegate::GetSourceTabID() const {
+  return sync_sessions::kInvalidTabID;
+}
+
 std::string IOSChromeSyncedTabDelegate::GetExtensionAppId() const {
   return std::string();
 }