Add experimental code behind a flag for Content Size Policy

content/browser/frame_host/navigation_handle_impl.cc

Index: content/browser/frame_host/navigation_handle_impl.cc
diff --git a/content/browser/frame_host/navigation_handle_impl.cc b/content/browser/frame_host/navigation_handle_impl.cc
index 47dc9a23962c4e42ceaede2f50cd7418f4283fe0..5279ecf2eae554eb394dd7638c5996d3afb181e9 100644
--- a/content/browser/frame_host/navigation_handle_impl.cc
+++ b/content/browser/frame_host/navigation_handle_impl.cc
@@ -11,6 +11,7 @@
 #include "content/browser/frame_host/frame_tree_node.h"
 #include "content/browser/frame_host/navigator.h"
 #include "content/browser/frame_host/navigator_delegate.h"
+#include "content/browser/loader/loader_io_thread_notifier.h"
 #include "content/browser/service_worker/service_worker_context_wrapper.h"
 #include "content/common/frame_messages.h"
 #include "content/common/resource_request_body_impl.h"
@@ -561,6 +562,11 @@ void NavigationHandleImpl::RegisterNavigationThrottles() {
       RenderFrameDevToolsAgentHost::CreateThrottleForNavigation(this);
   if (devtools_throttle)
     throttles_.push_back(devtools_throttle.release());
+
+  NavigationThrottle* content_size_throttle =
+      LoaderIOThreadNotifier::MaybeCreateThrottle(this);
+  if (content_size_throttle)
+    throttles_.push_back(base::WrapUnique(content_size_throttle));
 }
 
 }  // namespace content