With --site-per-process, avoid a crash when a subframe process goes away.

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 faafe5fb49ca385998d05dc7ed188b21e19d26fd..db90529d248664b8ab0cb041402a1c3a6369117a 100644
--- a/content/browser/frame_host/frame_tree_node.cc
+++ b/content/browser/frame_host/frame_tree_node.cc
@@ -72,13 +72,13 @@ void FrameTreeNode::RemoveChild(FrameTreeNode* child) {
   }
 }
 
-void FrameTreeNode::ResetForMainFrameSwap() {
+void FrameTreeNode::ResetForNewProcess() {
   frame_id_ = kInvalidFrameId;
   current_url_ = GURL();
 
   // The children may not have been cleared if a cross-process navigation
   // commits before the old process cleans everything up.  Make sure the child
-  // nodes get deleted.
+  // nodes get deleted before swapping to a new process.
   children_.clear();
 }