Refactor serialisation of openerId & openerNavigationIndex.

ios/chrome/browser/tabs/tab.h

Index: ios/chrome/browser/tabs/tab.h
diff --git a/ios/chrome/browser/tabs/tab.h b/ios/chrome/browser/tabs/tab.h
index 14acb25a36751e499b1cdd6b9356af68082a9a22..62b5e39acadf70e558f4113c6e828305d251b188 100644
--- a/ios/chrome/browser/tabs/tab.h
+++ b/ios/chrome/browser/tabs/tab.h
@@ -116,9 +116,15 @@ extern NSString* const kProxyPassthroughHeaderValue;
 @property(nonatomic, readonly) NSString* urlDisplayString;
 @property(nonatomic, readonly) NSString* windowName;
 
-// ID associated with this tab, from the SessionManager.
+// ID associated with this tab.
 @property(nonatomic, readonly) NSString* tabId;
 
+// ID of the opener of this tab.
+@property(nonatomic, readonly) NSString* openerID;
+
+// NavigationIndex of the opener of this tab.
+@property(nonatomic, readonly) NSInteger openerNavigationIndex;
+
 // |YES| if snapshot overlay should load from the grey image cache.
 @property(nonatomic, assign) BOOL useGreyImageCache;