Bug 814723 - Don't warn about missing 'browser.newtab.url' in nsDocSh…

…ell. r=jlebar

The preference key 'browser.newtab.url' is only supported for the browser,
and undefined on all other targets; such as mobile or b2g. For those targets
the function 'nsDocShell::ShouldAddToSessionHistory' warns about the missing
key.

This patch silences this warning by making Gecko return a default value of
'true' if the key is not present in the preferences.

docshell/base/nsDocShell.cpp

@@ -10316,10 +10316,13 @@ nsDocShell::ShouldAddToSessionHistory(nsIURI * aURI)
         }
     }
 
-    rv = aURI->GetSpec(buf);
-    NS_ENSURE_SUCCESS(rv, true);
-
     rv = Preferences::GetDefaultCString("browser.newtab.url", &pref);
+
+    if (NS_FAILED(rv)) {
+        return true;
+    }
+
+    rv = aURI->GetSpec(buf);
     NS_ENSURE_SUCCESS(rv, true);
 
     return !buf.Equals(pref);