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 7764e1642af36dec9e6ea33e55aa2f1fead90962..62642581816ac3ea9b1709c208e9a6801fb556b2 100644
--- a/chrome/browser/android/offline_pages/prerender_adapter.cc
+++ b/chrome/browser/android/offline_pages/prerender_adapter.cc
@@ -41,13 +41,17 @@ bool PrerenderAdapter::StartPrerender(
       prerender::PrerenderManagerFactory::GetForBrowserContext(browser_context);
   DCHECK(manager);
 
-  // Start prerendering the url and capture the handle for the prerendering.
+  // AddPrerenderForOffline() triggers the start event, and it 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;
 }
 
@@ -80,8 +84,8 @@ void PrerenderAdapter::DestroyActive() {
 }
 
 void PrerenderAdapter::OnPrerenderStart(prerender::PrerenderHandle* handle) {
-  DCHECK_EQ(active_handle_.get(), handle);
-  observer_->OnPrerenderStart();
+  // Not expected as the start event will happen before the observer is set.
+  NOTREACHED();
 }
 
 void PrerenderAdapter::OnPrerenderStopLoading(