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

content/browser/frame_host/frame_tree.h

Index: content/browser/frame_host/frame_tree.h
diff --git a/content/browser/frame_host/frame_tree.h b/content/browser/frame_host/frame_tree.h
index fb29df24de0d0fd5ac393981965fe01a8499876e..4af9215ab4d430f1de3e4283cee7c8c5a19d062e 100644
--- a/content/browser/frame_host/frame_tree.h
+++ b/content/browser/frame_host/frame_tree.h
@@ -60,7 +60,9 @@ class CONTENT_EXPORT FrameTree {
 
   // Executes |on_node| on each node in the frame tree.  If |on_node| returns
   // false, terminates the iteration immediately. Returning false is useful
-  // if |on_node| is just doing a search over the tree.
+  // if |on_node| is just doing a search over the tree.  The iteration proceeds
+  // top-down and visits a node before adding its children to the queue, making
+  // it safe to remove children during the callback.
   void ForEach(const base::Callback<bool(FrameTreeNode*)>& on_node) const;
 
   // After the FrameTree is created, or after SwapMainFrame() has been called,
@@ -92,6 +94,13 @@ class CONTENT_EXPORT FrameTree {
   // TODO(creis): Look into how we can remove the need for this method.
   void ResetForMainFrameSwap();
 
+  // Update the frame tree after a process exits.  Any nodes currently using the
+  // given |render_view_host| will lose all their children.
+  // TODO(creis): This should take a RenderProcessHost once RenderFrameHost
+  // knows its process.  Until then, we would just be asking the RenderViewHost
+  // for its process, so we'll skip that step.
+  void RenderProcessGone(RenderViewHost* render_view_host);
+
   // Convenience accessor for the main frame's RenderFrameHostImpl.
   RenderFrameHostImpl* GetMainFrame() const;