Add a custom element to own structure of shadow DOM plugin placeholders.

Source/core/html/shadow/PluginPlaceholderElement.js

-/**
- * This is a placeholder to create the
- * IDR_PRIVATE_SCRIPT_PLUGINPLACEHOLDERELEMENT_JS resource.
- *
- * It will be replaced by a complete file in:
- *   https://codereview.chromium.org/522783002/
- */
+// 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.createdCallback = function() {
+        // FIXME: Move style out of script and into CSS.
+
+        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.textContent; },
+        set: function(message) { this.messageElement.textContent = message; },
+    });
+});