Revert of Revert of OOPIF: Specify previous sibling frames when creating new RenderFrames.

content/browser/frame_host/frame_tree_node.cc

Index: content/browser/frame_host/frame_tree_node.cc
diff --git a/content/browser/frame_host/frame_tree_node.cc b/content/browser/frame_host/frame_tree_node.cc
index 67a0da519d8eccc22deb30938279cb9b8a4684f7..7403ea3c9f0572fb51a8eb0605bda46449cd56e9 100644
--- a/content/browser/frame_host/frame_tree_node.cc
+++ b/content/browser/frame_host/frame_tree_node.cc
@@ -159,6 +159,19 @@
   return false;
 }
 
+FrameTreeNode* FrameTreeNode::PreviousSibling() const {
+  if (!parent_)
+    return nullptr;
+
+  for (size_t i = 0; i < parent_->child_count(); ++i) {
+    if (parent_->child_at(i) == this)
+      return (i == 0) ? nullptr : parent_->child_at(i - 1);
+  }
+
+  NOTREACHED() << "FrameTreeNode not found in its parent's children.";
+  return nullptr;
+}
+
 bool FrameTreeNode::IsLoading() const {
   RenderFrameHostImpl* current_frame_host =
       render_manager_.current_frame_host();