Don't add an extension tab twice when it has an associated tab contents

This was deleted by accident in https://chromiumcodereview.appspot.com/10868116/

BUG=161854
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Review URL: https://chromiumcodereview.appspot.com/11348150

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@168798 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/chrome/browser/extensions/extension_host.cc b/chrome/browser/extensions/extension_host.cc
index 7d64bbfa..8ef2f92c 100644
--- a/chrome/browser/extensions/extension_host.cc
+++ b/chrome/browser/extensions/extension_host.cc
@@ -626,6 +626,7 @@
         delegate->AddNewContents(
             associated_contents, new_contents, disposition, initial_pos,
             user_gesture, was_blocked);
+        return;
       }
     }
   }