Notify the Blink client synchronously if the initial doc is accessed.

content/renderer/render_frame_impl.cc

Index: content/renderer/render_frame_impl.cc
diff --git a/content/renderer/render_frame_impl.cc b/content/renderer/render_frame_impl.cc
index d2030c35e017a83ed17e6526dbb2907d173b90c1..be5d4da59db647458e1ce3322f9852a9fffe7b6a 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -2776,6 +2776,9 @@ RenderFrameImpl::createServiceWorkerProvider() {
 }
 
 void RenderFrameImpl::didAccessInitialDocument() {
+  // NOTE: It is critical not to call back into JavaScript here, since this call
+  // is made from a V8 security check.
+
   // If the request hasn't yet committed, notify the browser process that it is
   // no longer safe to show the pending URL of the main frame, since a URL spoof
   // is now possible. (If the request has committed, the browser already knows.)