Move plugin placeholder style to CSS, and allow it to bypass main world CSP.

Source/core/html/shadow/PluginPlaceholderElement.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 'use strict';
 
 installClass('PluginPlaceholderElement', function(PluginPlaceholderElementPrototype) {
     PluginPlaceholderElementPrototype.createCallback = function() {
         // Produces DOM roughly equivalent to the following HTML:
         //
         // <div id="plugin-placeholder">
         //   <div id="plugin-placeholder-content">
         //     <div id="plugin-placeholder-message"></div>
         //   </div>
         // </div>
-        //
-        // FIXME: Move style out of script and into CSS.
+
+        this.appendChild(this.createStyleElement());
 
         this.id = 'plugin-placeholder';
-        this.style.width = '100%';
-        this.style.height = '100%';
-        this.style.overflow = 'hidden';
-        this.style.display = 'flex';
-        this.style.alignItems = 'center';
-        this.style.backgroundColor = 'gray';
-        this.style.font = '12px -webkit-control';
 
         var contentElement = document.createElement('div');
         contentElement.id = 'plugin-placeholder-content';
-        contentElement.style.textAlign = 'center';
-        contentElement.style.margin = 'auto';
 
         var messageElement = document.createElement('div');
         messageElement.id = 'plugin-placeholder-message';
 
         contentElement.appendChild(messageElement);
         this.appendChild(contentElement);
 
         this.messageElement = messageElement;
     };
 
     Object.defineProperty(PluginPlaceholderElementPrototype, 'message', {
         get: function() { return this.messageElement.innerHTML; },
         set: function(html) { this.messageElement.innerHTML = html; },
     });
 });