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

Source/core/html/shadow/PluginPlaceholderElement.css

Index: Source/core/html/shadow/PluginPlaceholderElement.css
diff --git a/Source/core/html/shadow/PluginPlaceholderElement.css b/Source/core/html/shadow/PluginPlaceholderElement.css
new file mode 100644
index 0000000000000000000000000000000000000000..23fb8bc25c70401add5e9f910c8d36a3d38661ae
--- /dev/null
+++ b/Source/core/html/shadow/PluginPlaceholderElement.css
@@ -0,0 +1,20 @@
+/*
+ * 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.
+ */
+
+#plugin-placeholder {
+    width: 100%;
+    height: 100%;
+    overflow: hidden;
+    display: flex;
+    align-items: center;
+    background: gray;
+    font: 12px -webkit-control;
+}
+
+#plugin-placeholder-content {
+    text-align: center;
+    margin: auto;
+}