Do conservative prerendering based on the LocalPredictor in Dev/Canary.

chrome/browser/prerender/prerender_tab_helper.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_tab_helper.h"
 
 #include "base/metrics/histogram.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/time.h"
 #include "chrome/browser/predictors/logged_in_predictor_table.h"
+#include "chrome/browser/prerender/prerender_field_trial.h"
 #include "chrome/browser/prerender/prerender_histograms.h"
 #include "chrome/browser/prerender/prerender_manager.h"
 #include "chrome/browser/prerender/prerender_manager_factory.h"
 #include "chrome/browser/profiles/profile.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/navigation_details.h"
 #include "content/public/browser/navigation_entry.h"
 #include "content/public/browser/render_view_host.h"
 #include "content/public/browser/web_contents.h"
 #include "content/public/browser/web_contents_view.h"
@@ skipping 124 matching lines @@
     content::RenderViewHost* render_view_host) {
   url_ = url;
   RecordEvent(EVENT_MAINFRAME_CHANGE);
   RecordEventIfLoggedInURL(EVENT_MAINFRAME_CHANGE_DOMAIN_LOGGED_IN, url);
   PrerenderManager* prerender_manager = MaybeGetPrerenderManager();
   if (!prerender_manager)
     return;
   if (prerender_manager->IsWebContentsPrerendering(web_contents(), NULL))
     return;
   prerender_manager->MarkWebContentsAsNotPrerendered(web_contents());
+  if (IsLocalPredictorEnabled())

[Shishir, 2013/05/07 22:57:30]

In the previous line you are marking the web_contents as not prerendering and
then trying to prerender it. Looks like the prerender manager will be
inconsistent. Also is ProvisionalChangeToMainFrameUrl the right place to do this
swap? We currently do it in chrome::Navigate.

[tburkard, 2013/05/07 23:21:59]

No, this only means that the data currently in the webcontents was not
prerendered. This is done because the URL changed. If we swap in, we will update
this to say that it was prerendered, based on the swap-in. So this is correct
(in particular, because of the order).

+    prerender_manager->MaybeUsePrerenderedPage(web_contents(), url);
 }
 
 void PrerenderTabHelper::DidCommitProvisionalLoadForFrame(
     int64 frame_id,
     bool is_main_frame,
     const GURL& validated_url,
     content::PageTransition transition_type,
     content::RenderViewHost* render_view_host) {
   if (!is_main_frame)
     return;
   RecordEvent(EVENT_MAINFRAME_COMMIT);
   RecordEventIfLoggedInURL(EVENT_MAINFRAME_COMMIT_DOMAIN_LOGGED_IN,
                            validated_url);
   url_ = validated_url;
   PrerenderManager* prerender_manager = MaybeGetPrerenderManager();
   if (!prerender_manager)
     return;
   if (prerender_manager->IsWebContentsPrerendering(web_contents(), NULL))
     return;
   prerender_manager->RecordNavigation(validated_url);
+  if (IsLocalPredictorEnabled())

[Shishir, 2013/05/07 22:57:30]

Why are we trying to swap at both start and commit of provisional load? Isn't
just one enough?

[tburkard, 2013/05/07 23:21:59]

Some URL changes only show up in one or the other, but not in both, which is why
we need this twice.

[Shishir, 2013/05/08 20:19:20]

Can you mention which ones show up on in only one or other? Isn't the commit of
a load a bit late to swap in prerendering. It possibly means that we have
recieved a response for the request. 

[tburkard, 2013/05/08 20:35:47]

Removing these changes in this file altogether, since Chris is preparing a new
change to address this that is cleaner.

+    prerender_manager->MaybeUsePrerenderedPage(web_contents(), validated_url);
 }
 
 void PrerenderTabHelper::DidStopLoading(
     content::RenderViewHost* render_view_host) {
   // Compute the PPLT metric and report it in a histogram, if needed.
   // We include pages that are still prerendering and have just finished
   // loading -- PrerenderManager will sort this out and handle it correctly
   // (putting those times into a separate histogram).
   if (!pplt_load_start_.is_null()) {
     double fraction_elapsed_at_swapin = -1.0;
@@ skipping 112 matching lines @@
 
 void PrerenderTabHelper::RecordEventIfLoggedInURLResult(
     PrerenderTabHelper::Event event,
     scoped_ptr<bool> is_present,
     scoped_ptr<bool> lookup_succeeded) {
   if (*lookup_succeeded && *is_present)
     RecordEvent(event);
 }
 
 }  // namespace prerender