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

Source/core/html/shadow/PluginPlaceholderElement.js

Index: Source/core/html/shadow/PluginPlaceholderElement.js
diff --git a/Source/core/html/shadow/PluginPlaceholderElement.js b/Source/core/html/shadow/PluginPlaceholderElement.js
index 70067ca35468f31b3d0042ca44708663e63eac56..a760d60cb57aa7763fdbc9984b5fe0360067d265 100644
--- a/Source/core/html/shadow/PluginPlaceholderElement.js
+++ b/Source/core/html/shadow/PluginPlaceholderElement.js
@@ -13,22 +13,13 @@ installClass('PluginPlaceholderElement', function(PluginPlaceholderElementProtot
         //     <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';