Fix issues when PrerenderContents::Destroy() is called during TabContents destruction.

chrome/browser/prerender/prerender_contents.cc

Index: chrome/browser/prerender/prerender_contents.cc
===================================================================
--- chrome/browser/prerender/prerender_contents.cc	(revision 85626)
+++ chrome/browser/prerender/prerender_contents.cc	(working copy)
@@ -682,10 +682,9 @@
 }
 
 void PrerenderContents::Destroy(FinalStatus final_status) {
-  if (prerender_manager_->IsPendingDelete(this))
+  if (!prerender_manager_->MoveEntryToPendingDelete(this))
     return;
 
-  prerender_manager_->MoveEntryToPendingDelete(this);
   set_final_status(final_status);
   // We may destroy the PrerenderContents before we have initialized the
   // RenderViewHost. Otherwise set the Observer's PrerenderContents to NULL to