Fix to prevent hitting NOTREACHED in FrameTreeNode::PreviousSibling().

content/browser/frame_host/frame_tree_node.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/frame_host/frame_tree_node.h"
 
 #include <queue>
 #include <utility>
 
 #include "base/macros.h"
@@ skipping 103 matching lines @@
 
   RecordUniqueNameLength(unique_name.size());
   TRACE_EVENT_OBJECT_CREATED_WITH_ID(
       "navigation", "FrameTreeNode",
       TRACE_ID_WITH_SCOPE("FrameTreeNode", frame_tree_node_id_));
   // Don't TraceSnapshot() until the RenderFrameHostManager is initialized and
   // calls SetCurrentURL().
 }
 
 FrameTreeNode::~FrameTreeNode() {
-  children_.clear();
+  std::vector<std::unique_ptr<FrameTreeNode>>().swap(children_);
   frame_tree_->FrameRemoved(this);
   FOR_EACH_OBSERVER(Observer, observers_, OnFrameTreeNodeDestroyed(this));
 
   if (opener_)
     opener_->RemoveObserver(opener_observer_.get());
 
   g_frame_tree_node_id_map.Get().erase(frame_tree_node_id_);
 
   TRACE_EVENT_OBJECT_DELETED_WITH_ID(
       "navigation", "FrameTreeNode",
@@ skipping 348 matching lines @@
 void FrameTreeNode::TraceSnapshot() const {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   TRACE_EVENT_OBJECT_SNAPSHOT_WITH_ID(
       "navigation", "FrameTreeNode",
       TRACE_ID_WITH_SCOPE("FrameTreeNode", frame_tree_node_id_),
       std::unique_ptr<base::trace_event::ConvertableToTraceFormat>(
           new TracedFrameTreeNode(*this)));
 }
 
 FrameTreeNode* FrameTreeNode::GetSibling(int relative_offset) const {
-  if (!parent_)
+  if (!parent_ || !parent_->child_count())
     return nullptr;
 
   for (size_t i = 0; i < parent_->child_count(); ++i) {
     if (parent_->child_at(i) == this) {
       if ((relative_offset < 0 && static_cast<size_t>(-relative_offset) > i) ||
           i + relative_offset >= parent_->child_count()) {
         return nullptr;
       }
       return parent_->child_at(i + relative_offset);
     }
   }
 
   NOTREACHED() << "FrameTreeNode not found in its parent's children.";
   return nullptr;
 }
 
 }  // namespace content