bug 1210447 - fix ProxyAccessible::EmbeddedChildAt() on proxies of outerDocs r=davidb

accessible/ipc/ProxyAccessible.cpp

@@ -1000,6 +1000,14 @@ ProxyAccessible::IndexOfEmbeddedChild(const ProxyAccessible* aChild)
 ProxyAccessible*
 ProxyAccessible::EmbeddedChildAt(size_t aChildIdx)
 {
+  // For an outer doc the only child is a document, which is of course an
+  // embedded child.  Further asking the child process for the id of the child
+  // document won't work because the id of the child doc will be 0, which we
+  // would interpret as being our parent document.
+  if (mOuterDoc) {
+    return ChildAt(aChildIdx);
+  }
+
   uint64_t childID;
   unused << mDoc->SendEmbeddedChildAt(mID, aChildIdx, &childID);
   return mDoc->GetAccessible(childID);