Bug 1373672 - Part 3: Expose childOffset from nsIDocShell to use in nsSessionStoreUtils, r=ttaubert, r=smaug

The reasoning behind this is that with this change, removing a non-dynamic
docshell from the document dynamically shouldn't affect the indexes which we use
for both recording and restoring data in child docshells.

MozReview-Commit-ID: JIK8GBSWDEF

docshell/base/nsDocShell.cpp

@@ -4089,6 +4089,13 @@ nsDocShell::SetChildOffset(int32_t aChildOffset)
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsDocShell::GetChildOffset(int32_t* aChildOffset)
+{
+  *aChildOffset = mChildOffset;
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsDocShell::GetHistoryID(nsID** aID)
 {