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..5929cb5fc2d112758461495fae0f891795c3b358 100644
--- a/chrome/browser/android/offline_pages/prerender_adapter.cc
+++ b/chrome/browser/android/offline_pages/prerender_adapter.cc
@@ -42,12 +42,18 @@ bool PrerenderAdapter::StartPrerender(
   DCHECK(manager);
 
   // Start prerendering the url and capture the handle for the prerendering.
+  // Note: PrerenderHandle::OnPrerenderStart() will get called inside

[dougarnett, 2016/09/21 17:01:27]

maybe drop this comment with something similar by new NOTREACHED call

[qinmin, 2016/09/21 17:18:01]

Done.

+  // 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;
 }
 
@@ -80,8 +86,7 @@ void PrerenderAdapter::DestroyActive() {
 }
 
 void PrerenderAdapter::OnPrerenderStart(prerender::PrerenderHandle* handle) {
-  DCHECK_EQ(active_handle_.get(), handle);
-  observer_->OnPrerenderStart();
+  NOTREACHED();

[dougarnett, 2016/09/21 17:01:27]

maybe some comment here instead, eg,

// Not expected as the start event will happen before the observer is set.

[qinmin, 2016/09/21 17:18:01]

Done.

 }
 
 void PrerenderAdapter::OnPrerenderStopLoading(