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

chrome/browser/android/offline_pages/prerendering_loader_unittest.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/android/offline_pages/prerendering_loader.h"
 
 #include "base/bind.h"
 #include "base/memory/ptr_util.h"
 #include "base/memory/ref_counted.h"
 #include "base/run_loop.h"
@@ skipping 200 matching lines @@
   test_adapter()->Configure(
       content::WebContentsTester::CreateTestWebContents(profile(), NULL),
       prerender::FinalStatus::FINAL_STATUS_USED);
   GURL gurl("http://testit.sea");
   EXPECT_TRUE(loader()->IsIdle());
   EXPECT_FALSE(loader()->IsLoaded());
   EXPECT_TRUE(loader()->LoadPage(
       gurl,
       base::Bind(&PrerenderingLoaderTest::OnLoadDone, base::Unretained(this))));
 
-  test_adapter()->GetObserver()->OnPrerenderStart();
   PumpLoop();
   EXPECT_FALSE(loader()->IsIdle());
   EXPECT_FALSE(loader()->IsLoaded());
 
   test_adapter()->GetObserver()->OnPrerenderStopLoading();
   PumpLoop();
   EXPECT_FALSE(loader()->IsIdle());
   EXPECT_TRUE(loader()->IsLoaded());
   EXPECT_TRUE(callback_called());
   EXPECT_EQ(Offliner::RequestStatus::LOADED, callback_load_status());
@@ skipping 103 matching lines @@
   test_adapter()->FailStart();
   GURL gurl("http://testit.sea");
   EXPECT_TRUE(loader()->IsIdle());
   EXPECT_FALSE(loader()->LoadPage(
       gurl,
       base::Bind(&PrerenderingLoaderTest::OnLoadDone, base::Unretained(this))));
   EXPECT_TRUE(loader()->IsIdle());
 }
 
 }  // namespace offline_pages