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

content/browser/frame_host/frame_tree.cc

Index: content/browser/frame_host/frame_tree.cc
diff --git a/content/browser/frame_host/frame_tree.cc b/content/browser/frame_host/frame_tree.cc
index c135da80d340fb2c3a90bdb3c57d138cc2fbfd91..0b3275a25ac1b7305c92fa418207391dc2fcadc0 100644
--- a/content/browser/frame_host/frame_tree.cc
+++ b/content/browser/frame_host/frame_tree.cc
@@ -43,6 +43,15 @@ bool FrameTreeNodeForFrameId(int64 frame_id,
   return true;
 }
 
+// Iterate over the FrameTree to reset any node affected by the loss of the
+// given RenderViewHost's process.
+bool ResetNodesForNewProcess(RenderViewHost* render_view_host,
+                             FrameTreeNode* node) {
+  if (render_view_host == node->current_frame_host()->render_view_host())
+    node->ResetForNewProcess();
+  return true;
+}
+
 }  // namespace
 
 FrameTree::FrameTree(Navigator* navigator,
@@ -153,7 +162,17 @@ void FrameTree::SetFrameUrl(int64 frame_id, const GURL& url) {
 }
 
 void FrameTree::ResetForMainFrameSwap() {
-  return root_->ResetForMainFrameSwap();
+  root_->ResetForNewProcess();
+}
+
+void FrameTree::RenderProcessGone(RenderViewHost* render_view_host) {
+  // Walk the full tree looking for nodes that may be affected.  Once a frame
+  // crashes, all of its child FrameTreeNodes go away.
+  // Note that the helper function may call ResetForNewProcess on a node, which
+  // clears its children before we iterate over them.  That's ok, because
+  // ForEach does not add a node's children to the queue until after visiting
+  // the node itself.
+  ForEach(base::Bind(&ResetNodesForNewProcess, render_view_host));
 }
 
 RenderFrameHostImpl* FrameTree::GetMainFrame() const {