Create a new RenderFrameHost per child frame when --site-per-process is enabled.

content/browser/renderer_host/render_view_host_delegate.h

Index: content/browser/renderer_host/render_view_host_delegate.h
diff --git a/content/browser/renderer_host/render_view_host_delegate.h b/content/browser/renderer_host/render_view_host_delegate.h
index aa3d70425ae9ba62f90980395c04182b0b13cf3f..eb20d2bee51f8fcce4537f0d9817474ff008348b 100644
--- a/content/browser/renderer_host/render_view_host_delegate.h
+++ b/content/browser/renderer_host/render_view_host_delegate.h
@@ -46,6 +46,7 @@ class Size;
 namespace content {
 
 class BrowserContext;
+class FrameTree;
 class PageState;
 class RenderViewHost;
 class RenderViewHostDelegateView;
@@ -432,6 +433,8 @@ class CONTENT_EXPORT RenderViewHostDelegate {
   virtual SessionStorageNamespace* GetSessionStorageNamespace(
       SiteInstance* instance);
 
+  virtual FrameTree* GetFrameTree();
+
  protected:
   virtual ~RenderViewHostDelegate() {}
 };