[Prerender] Add a "SimpleLoad" mode to be used as a comparison reference

chrome/browser/prerender/prerender_manager.cc

 // Copyright (c) 2012 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/prerender/prerender_manager.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <functional>
@@ skipping 595 matching lines @@
 // static
 bool PrerenderManager::IsPrerenderingPossible() {
   return GetMode() != PRERENDER_MODE_DISABLED;
 }
 
 // static
 bool PrerenderManager::IsNoStatePrefetch() {
   return GetMode() == PRERENDER_MODE_NOSTATE_PREFETCH;
 }
 
+// static
+bool PrerenderManager::IsSimpleLoadExperiment() {
+  return GetMode() == PRERENDER_MODE_SIMPLE_LOAD_EXPERIMENT;
+}
+
 bool PrerenderManager::IsWebContentsPrerendering(
     const WebContents* web_contents,
     Origin* origin) const {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   PrerenderContents* prerender_contents = GetPrerenderContents(web_contents);
   if (!prerender_contents)
     return false;
 
   if (origin)
     *origin = prerender_contents->origin();
@@ skipping 310 matching lines @@
   // Check if enough time has passed since the last prerender.
   if (!DoesRateLimitAllowPrerender(origin)) {
     // Cancel the prerender. We could add it to the pending prerender list but
     // this doesn't make sense as the next prerender request will be triggered
     // by a navigation and is unlikely to be the same site.
     RecordFinalStatusWithoutCreatingPrerenderContents(
         url, origin, FINAL_STATUS_RATE_LIMIT_EXCEEDED);
     return nullptr;
   }
 
+  // Record the URL in the prefetch list, even when in full prerender mode, to
+  // enable metrics comparisons.
+  prefetches_.emplace_back(url, GetCurrentTimeTicks(), origin);
+
+  if (IsSimpleLoadExperiment()) {
+    // Exit after adding the url to prefetches_, so that no prefetching occurs
+    // but the page is still tracked as "would have been prefetched".
+    return nullptr;
+  }
+
   std::unique_ptr<PrerenderContents> prerender_contents =
       CreatePrerenderContents(url, referrer, origin);
   DCHECK(prerender_contents);
   PrerenderContents* prerender_contents_ptr = prerender_contents.get();
   if (IsNoStatePrefetch())
     prerender_contents_ptr->SetPrerenderMode(PREFETCH_ONLY);
   active_prerenders_.push_back(
       base::MakeUnique<PrerenderData>(this, std::move(prerender_contents),
                                       GetExpiryTimeForNewPrerender(origin)));
   if (!prerender_contents_ptr->Init()) {
     DCHECK(active_prerenders_.end() ==
            FindIteratorForPrerenderContents(prerender_contents_ptr));
     return nullptr;
   }
 
   histograms_->RecordPrerenderStarted(origin);
   DCHECK(!prerender_contents_ptr->prerendering_has_started());
 
-  if (prerender_contents_ptr->prerender_mode() == PREFETCH_ONLY)
-    prefetches_.emplace_back(url, GetCurrentTimeTicks(), origin);
-
   std::unique_ptr<PrerenderHandle> prerender_handle =
       base::WrapUnique(new PrerenderHandle(active_prerenders_.back().get()));
   SortActivePrerenders();
 
   last_prerender_start_time_ = GetCurrentTimeTicks();
 
   gfx::Rect contents_bounds =
       bounds.IsEmpty() ? config_.default_tab_bounds : bounds;
 
   prerender_contents_ptr->StartPrerendering(contents_bounds,
@@ skipping 372 matching lines @@
   return weak_factory_.GetWeakPtr();
 }
 
 void PrerenderManager::SetPrerenderContentsFactoryForTest(
     PrerenderContents::Factory* prerender_contents_factory) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   prerender_contents_factory_.reset(prerender_contents_factory);
 }
 
 }  // namespace prerender