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

chrome/browser/android/offline_pages/prerender_adapter.h

Index: chrome/browser/android/offline_pages/prerender_adapter.h
diff --git a/chrome/browser/android/offline_pages/prerender_adapter.h b/chrome/browser/android/offline_pages/prerender_adapter.h
index 1382170565df9b1bab5da23d274f07d8365de9e3..bb5f0a78e4db6ec6c4e24aad9c8c23b9eef6c25a 100644
--- a/chrome/browser/android/offline_pages/prerender_adapter.h
+++ b/chrome/browser/android/offline_pages/prerender_adapter.h
@@ -30,9 +30,6 @@ class PrerenderAdapter : public prerender::PrerenderHandle::Observer {
   // An observer of PrerenderHandle events that does not expose the handle.
   class Observer {
    public:
-    // Signals that the prerender has started running.
-    virtual void OnPrerenderStart() = 0;
-
     // Signals that the prerender has had its load event.
     virtual void OnPrerenderStopLoading() = 0;