Replace PruneAllButActive with PruneAllButVisible.

chrome/browser/ui/browser_instant_controller.cc

Index: chrome/browser/ui/browser_instant_controller.cc
diff --git a/chrome/browser/ui/browser_instant_controller.cc b/chrome/browser/ui/browser_instant_controller.cc
index 34e1ee717c995cf64155a0d39972e9467d70396b..226f5a0eb981c63c9e3884699f3cca87be32d4de 100644
--- a/chrome/browser/ui/browser_instant_controller.cc
+++ b/chrome/browser/ui/browser_instant_controller.cc
@@ -94,19 +94,21 @@ bool BrowserInstantController::MaybeSwapInInstantNTPContents(
     return false;
   }
 
+  // Make sure the instant WebContents is in a valid state (i.e., has a last
+  // committed entry, no transient entry, and no existing pending entry).
   scoped_ptr<content::WebContents> instant_ntp = instant_.ReleaseNTPContents();
-  if (!instant_ntp)
+  if (!instant_ntp || !instant_ntp->GetController().CanPruneAllButVisible())
     return false;
 
   *target_contents = instant_ntp.get();
   if (source_contents) {
-    instant_ntp->GetController().CopyStateFromAndPrune(
-        &source_contents->GetController());
+    CHECK(instant_ntp->GetController().CopyStateFromAndPrune(
+              &source_contents->GetController()));
     ReplaceWebContentsAt(
         browser_->tab_strip_model()->GetIndexOfWebContents(source_contents),
         instant_ntp.Pass());
   } else {
-    instant_ntp->GetController().PruneAllButActive();
+    CHECK(instant_ntp->GetController().PruneAllButVisible());
     // If |source_contents| is NULL, then the caller is responsible for
     // inserting instant_ntp into the tabstrip and will take ownership.
     ignore_result(instant_ntp.release());