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

Source/core/html/shadow/PluginPlaceholderElement.h

Index: Source/core/html/shadow/PluginPlaceholderElement.h
diff --git a/Source/core/html/shadow/PluginPlaceholderElement.h b/Source/core/html/shadow/PluginPlaceholderElement.h
index b65f1f65d27d5aaa0a0f407c19c9c0cfd044df49..6f426c7a7a6fdf96faeafec9567817c641ad4407 100644
--- a/Source/core/html/shadow/PluginPlaceholderElement.h
+++ b/Source/core/html/shadow/PluginPlaceholderElement.h
@@ -10,6 +10,8 @@
 
 namespace blink {
 
+class HTMLStyleElement;
+
 class PluginPlaceholderElement FINAL : public HTMLDivElement {
 public:
     // Returns a null pointer if the placeholder DOM could not be built.
@@ -17,6 +19,9 @@ public:
 
     void setMessage(const String& htmlMessage);
 
+    // Exposed for private script.
+    PassRefPtrWillBeRawPtr<HTMLStyleElement> createStyleElement();
+
 private:
     explicit PluginPlaceholderElement(Document&);
 };