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

Source/core/html/shadow/PluginPlaceholderElement.h

 // 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.
 
 #ifndef PluginPlaceholderElement_h
 #define PluginPlaceholderElement_h
 
 #include "core/dom/Element.h"
 #include "core/html/HTMLDivElement.h"
 
 namespace blink {
 
+class HTMLStyleElement;
+
 class PluginPlaceholderElement FINAL : public HTMLDivElement {
 public:
     // Returns a null pointer if the placeholder DOM could not be built.
     static PassRefPtrWillBeRawPtr<PluginPlaceholderElement> create(Document&);
 
     void setMessage(const String& htmlMessage);
 
+    // Exposed for private script.
+    PassRefPtrWillBeRawPtr<HTMLStyleElement> createStyleElement();
+
 private:
     explicit PluginPlaceholderElement(Document&);
 };
 
 } // namespace blink
 
 #endif // PluginPlaceholderElement_h