[ios] Declare missing i variable in plugin_placeholder.js.

BUG=487804

Review-Url: https://codereview.chromium.org/2346703002
Cr-Commit-Position: refs/heads/master@{#419246}

ios/web/web_state/js/resources/plugin_placeholder.js

@@ -128,7 +128,7 @@ __gCrWeb['plugin'] = {};
    * @param {string} message The message to show in the placeholder.
    */
   __gCrWeb['plugin']['addPluginPlaceholders'] = function(message) {
-    var plugins = __gCrWeb['placeholderTargetPlugins'];
+    var i, plugins = __gCrWeb['placeholderTargetPlugins'];
     for (i = 0; i < plugins.length; i++) {
       var plugin = plugins[i];
       if (!__gCrWeb['plugin'].isSignificantPlugin_(plugin)) {