Verify we don't traverse the frame tree during its construction.

We shouldn't be able to create a traversal iterator when there's
no current_frame_host(), as it will crash in NodeIterator::operator++
when that function tries to access child_count() via |current_node_|.

Change-Id: Ib9314b4189d9828fa5ff22a58b89b414426be590
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867435
Commit-Queue: W. James MacLean <wjmaclean@google.com>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707245}

content/browser/frame_host/frame_tree.cc

@@ -78,6 +78,9 @@ FrameTree::NodeIterator::NodeIterator(FrameTreeNode* starting_node,
       root_of_subtree_to_skip_(root_of_subtree_to_skip) {}
 
 FrameTree::NodeIterator FrameTree::NodeRange::begin() {
+  // We shouldn't be attempting a frame tree traversal while the tree is
+  // being constructed.
+  DCHECK(root_->current_frame_host());
   return NodeIterator(root_, root_of_subtree_to_skip_);
 }