Change ComputeTopFrameOrigin to use GetLastCommittedOrigin

ComputeTopFrameOrigin currently uses current_origin(), which has a
prominent warning about possible incorrect behavior. The function's use
is likely causing incorrect Network Isolation Keys to be used. We thus
use the replacement suggested by the warning.

Bug: 1014077
Change-Id: Iaecbb0ab028c626dce037462bb22053b5114bc52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1860106Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Shivani Sharma <shivanisha@chromium.org>
Commit-Queue: Alex Turner <alexmt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706911}

content/browser/frame_host/render_frame_host_impl.cc

@@ -2239,9 +2239,16 @@ void RenderFrameHostImpl::SetLastCommittedOriginForTesting(
 
 const url::Origin& RenderFrameHostImpl::ComputeTopFrameOrigin(
     const url::Origin& frame_origin) const {
-  return frame_tree_node_->IsMainFrame()
-             ? frame_origin
-             : frame_tree_->root()->current_origin();
+  if (frame_tree_node_->IsMainFrame()) {
+    return frame_origin;
+  }
+
+  DCHECK(parent_);
+  RenderFrameHostImpl* host = parent_;
+  while (host->parent_) {
+    host = host->parent_;
+  }
+  return host->GetLastCommittedOrigin();
 }
 
 GURL RenderFrameHostImpl::ComputeSiteForCookiesForNavigation(

content/browser/frame_host/render_frame_host_impl.h

@@ -475,6 +475,8 @@ class CONTENT_EXPORT RenderFrameHostImpl
   // Return the http status code of the last committed navigation.
   int last_http_status_code() { return last_http_status_code_; }
 
+  // Returns |frame_origin| if this frame is the top (i.e. root) frame in the
+  // frame tree. Otherwise, it returns the top frame's origin.
   const url::Origin& ComputeTopFrameOrigin(
       const url::Origin& frame_origin) const;