Replace PruneAllButActive with PruneAllButVisible.

chrome/browser/prerender/prerender_manager.cc

Index: chrome/browser/prerender/prerender_manager.cc
diff --git a/chrome/browser/prerender/prerender_manager.cc b/chrome/browser/prerender/prerender_manager.cc
index ec50600105588bb40af2ce4460a311662e5220a8..124bd88477e1f5536ccbac9539d3d75dcebe568f 100644
--- a/chrome/browser/prerender/prerender_manager.cc
+++ b/chrome/browser/prerender/prerender_manager.cc
@@ -427,6 +427,14 @@ bool PrerenderManager::MaybeUsePrerenderedPage(WebContents* web_contents,
       prerender_data->contents()->prerender_contents()) {
     if (web_contents == new_web_contents)
       return false;  // Do not swap in to ourself.
+
+    // We cannot swap in if there is no last committed entry, because we would
+    // show a blank page under an existing entry from the current tab.  Even if
+    // there is a pending entry, it may not commit.
+    // TODO(creis): If there is a pending navigation and no last committed
+    // entry, we might be able to transfer the network request instead.

[cbentzel, 2013/05/17 00:32:15]

I'd like to add UMA to see how frequently this happens - possibly even by
canceling the prerender contents. I'm happy to do it in a followup CL. 

[Charlie Reis, 2013/05/17 23:50:21]

Yes, that's a good idea.  Just increment a stat here if there's no last
committed entry but there is a pending entry.  (I can do it in this CL if you
let me know what name you want.  There's a bunch for prerendering and I'm not
sure where this would fit in.)

[cbentzel, 2013/05/18 21:11:08]

For now, I would actually destroy the prerender and add a new final status - see
many examples below like FINAL_STATUS_WINDOW_OPENER.

But I think it's easier to do that in a followup CL to keep this small - that
will require final_status.[h|cc] changes, plus histograms.xml change, etc.

+    if (!new_web_contents->GetController().CanPruneAllButVisible())
+      return false;
   }
 
   // Do not use the prerendered version if there is an opener object.
@@ -520,8 +528,8 @@ bool PrerenderManager::MaybeUsePrerenderedPage(WebContents* web_contents,
   MarkWebContentsAsPrerendered(new_web_contents, prerender_contents->origin());
 
   // Merge the browsing history.
-  new_web_contents->GetController().CopyStateFromAndPrune(
-      &old_web_contents->GetController());
+  CHECK(new_web_contents->GetController().CopyStateFromAndPrune(
+            &old_web_contents->GetController()));
   CoreTabHelper::FromWebContents(old_web_contents)->delegate()->
       SwapTabContents(old_web_contents, new_web_contents);
   prerender_contents->CommitHistory(new_web_contents);