Send origin updates to frame proxies when a frame navigates to new origin.

content/common/frame_replication_state.h

Index: content/common/frame_replication_state.h
diff --git a/content/common/frame_replication_state.h b/content/common/frame_replication_state.h
index f6cfa9407d67fd5ac00486d0e8cdc4cc60c51288..77ec8596466b58167e9dac7c51996f68a90809e6 100644
--- a/content/common/frame_replication_state.h
+++ b/content/common/frame_replication_state.h
@@ -48,15 +48,42 @@ struct CONTENT_EXPORT FrameReplicationState {
   FrameReplicationState(const std::string& name);
   ~FrameReplicationState();
 
-  // Current serialized security origin of the frame. Unique origins are
-  // represented as the string "null" per RFC 6454.
+  // Current serialized security origin of the frame.  Unique origins are
+  // represented as the string "null" per RFC 6454.  This field is updated
+  // whenever a frame navigation commits.
+  //
+  // TODO(alexmos): For now, |origin| updates are immediately sent to all frame
+  // proxies when in --site-per-process mode. This isn't ideal, since Blink
+  // typically needs a proxy's origin only when performing security checks on
+  // the ancestors of a local frame.  So, as a future improvement, we could
+  // delay sending origin updates to proxies until they have a local descendant
+  // (if ever). This would reduce leaking a user's browsing history into a
+  // compromized renderer.
   url::Origin origin;
 
-  // Current sandbox flags of the frame.
+  // Current sandbox flags of the frame.  |sandbox_flags| are initialized for
+  // new child frames using the value of the <iframe> element's "sandbox"
+  // attribute. They are updated dynamically whenever a parent frame updates an
+  // <iframe>'s sandbox attribute via JavaScript.
+  //
+  // Updates to |sandbox_flags| are sent to proxies, but only after a
+  // subsequent navigation of the (sandboxed) frame, since the flags only take
+  // effect on navigation (see also FrameTreeNode::effective_sandbox_flags_).
+  // The proxies need updated flags so that they can be inherited properly if a
+  // proxy ever becomes a parent of a local frame.
   SandboxFlags sandbox_flags;
 
   // The assigned name of the frame. This name can be empty, unlike the unique
   // name generated internally in the DOM tree.
+  //
+  // |name| is set when a new child frame is created using the value of the
+  // <iframe> element's "name" attribute (see
+  // RenderFrameHostImpl::OnCreateChildFrame), and it is updated dynamically
+  // whenever a frame sets its window.name.
+  //
+  // |name| updates are immediately sent to all frame proxies (when in
+  // --site-per-process mode), so that other frames can look up or navigate a
+  // frame using its updated name (e.g., using window.open(url, frame_name)).
   std::string name;
 
   // TODO(alexmos): Eventually, this structure can also hold other state that