Use the RenderFrameHost SiteInstance when creating subframes.

This is a follow up on a comment from
https://chromium-review.googlesource.com/c/chromium/src/+/1098562

Bug: 609963
Change-Id: I8ba97bcbb8961f1dbacd4d3fe5c1766c44c20042
Reviewed-on: https://chromium-review.googlesource.com/c/1265023Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597299}

content/browser/frame_host/render_frame_host_impl.cc

@@ -1725,7 +1725,7 @@ FrameTreeNode* RenderFrameHostImpl::AddChild(
   // Initialize the RenderFrameHost for the new node.  We always create child
   // frames in the same SiteInstance as the current frame, and they can swap to
   // a different one if they navigate away.
-  child->render_manager()->Init(render_view_host()->GetSiteInstance(),
+  child->render_manager()->Init(GetSiteInstance(),
                                 render_view_host()->GetRoutingID(),
                                 frame_routing_id, MSG_ROUTING_NONE, false);