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

chrome/browser/android/offline_pages/prerendering_loader.cc

Index: chrome/browser/android/offline_pages/prerendering_loader.cc
diff --git a/chrome/browser/android/offline_pages/prerendering_loader.cc b/chrome/browser/android/offline_pages/prerendering_loader.cc
index bbc1fe0851ac8f1d9753ce70fbdbde02b3ff795a..5882974fb8236f72deb3a1ed1b401dd895a65459 100644
--- a/chrome/browser/android/offline_pages/prerendering_loader.cc
+++ b/chrome/browser/android/offline_pages/prerendering_loader.cc
@@ -59,6 +59,10 @@ bool PrerenderingLoader::LoadPage(const GURL& url,
   callback_ = callback;
   session_contents_.swap(new_web_contents);
   state_ = State::PENDING;

[dougarnett, 2016/09/21 16:16:51]

Could just set LOADING here instead and drop the OnPrerenderStart logic and
maybe method.

[dougarnett, 2016/09/21 16:32:40]

BTW, I could do that clean up as it is a bit more involved. 

[qinmin, 2016/09/21 16:40:04]

Can you take another look at the new patch, I have cleaned up the
OnPrerenderStart() method

+  // Note: Observer is attached too late when calling
+  // PrerenderAdapter::StartPrerenderer(). Call the missing OnPrerenderStart()
+  // here.
+  OnPrerenderStart();
   return true;
 }