Add DCHECK to show that plznav sometimes provides empty global request ids.

chrome/browser/page_load_metrics/page_load_tracker.cc

Index: chrome/browser/page_load_metrics/page_load_tracker.cc
diff --git a/chrome/browser/page_load_metrics/page_load_tracker.cc b/chrome/browser/page_load_metrics/page_load_tracker.cc
index 6f161e9fa3d7913d4eba92157bb0c03939b0b0c4..7312aade22ba6a0ddb84067614611234cae1a38d 100644
--- a/chrome/browser/page_load_metrics/page_load_tracker.cc
+++ b/chrome/browser/page_load_metrics/page_load_tracker.cc
@@ -431,6 +431,7 @@ void PageLoadTracker::WillProcessNavigationResponse(
   // PlzNavigate: NavigationHandle::GetGlobalRequestID() sometimes returns an
   // uninitialized GlobalRequestID. Bail early in this case. See
   // crbug.com/680841 for details.
+  DCHECK(navigation_handle->GetGlobalRequestID() != content::GlobalRequestID());
   if (content::IsBrowserSideNavigationEnabled() &&
       navigation_handle->GetGlobalRequestID() == content::GlobalRequestID())
     return;