Switch AlternateErrorPageTabObserver to use WebContentsUserData.

chrome/browser/ui/tab_contents/tab_contents.cc

Index: chrome/browser/ui/tab_contents/tab_contents.cc
diff --git a/chrome/browser/ui/tab_contents/tab_contents.cc b/chrome/browser/ui/tab_contents/tab_contents.cc
index b31c44560e490675fa6c2a908e73ded6ed0a9c34..5c24358a9fed716f68b0dcdef0ea1e8a77d82d18 100644
--- a/chrome/browser/ui/tab_contents/tab_contents.cc
+++ b/chrome/browser/ui/tab_contents/tab_contents.cc
@@ -114,6 +114,7 @@ TabContents::TabContents(WebContents* contents)
   // helpers may rely on that.
   SessionTabHelper::CreateForWebContents(contents);
 
+  AlternateErrorPageTabObserver::CreateForWebContents(contents);
   autocomplete_history_manager_.reset(new AutocompleteHistoryManager(contents));
   autofill_delegate_.reset(new TabAutofillManagerDelegate(this));
   autofill_manager_ = new AutofillManager(autofill_delegate_.get(), this);
@@ -167,9 +168,6 @@ TabContents::TabContents(WebContents* contents)
   sad_tab_helper_.reset(new SadTabHelper(contents));
 #endif
 
-  // Create the per-tab observers.
-  alternate_error_page_tab_observer_.reset(
-      new AlternateErrorPageTabObserver(contents, profile()));
   external_protocol_observer_.reset(new ExternalProtocolObserver(contents));
   navigation_metrics_recorder_.reset(new NavigationMetricsRecorder(contents));
   pdf_tab_observer_.reset(new PDFTabObserver(this));