Fix an issue that PrerenderLoader::OnPrerenderStart() never gets called.

chrome/browser/android/offline_pages/prerender_adapter.cc

Index: chrome/browser/android/offline_pages/prerender_adapter.cc
diff --git a/chrome/browser/android/offline_pages/prerender_adapter.cc b/chrome/browser/android/offline_pages/prerender_adapter.cc
index 38bc605e006cf6c19ff1cb8243d704feaba85dfa..32efc65480a28b3fe8c5cae2bd8bb8eba25807c0 100644
--- a/chrome/browser/android/offline_pages/prerender_adapter.cc
+++ b/chrome/browser/android/offline_pages/prerender_adapter.cc
@@ -44,12 +44,18 @@ bool PrerenderAdapter::StartPrerender(
   DCHECK(manager);
 
   // Start prerendering the url and capture the handle for the prerendering.
+  // Note: PrerenderHandle::OnPrerenderStart() will get called inside
+  // PrerenderManager::AddPrerenderForOffline(), this is before the observer is
+  // set.
   active_handle_ =
       manager->AddPrerenderForOffline(url, session_storage_namespace, size);
   if (!active_handle_)
     return false;
+  DCHECK(active_handle_->contents());
+  DCHECK(active_handle_->contents()->prerendering_has_started());
 
   active_handle_->SetObserver(this);
+
   return true;
 }