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

Source/core/css/pluginPlaceholder.css

Index: Source/core/css/pluginPlaceholder.css
diff --git a/Source/core/css/pluginPlaceholder.css b/Source/core/css/pluginPlaceholder.css
new file mode 100644
index 0000000000000000000000000000000000000000..0b41f4024d2160b89a5aa07b265bb28830c279b5
--- /dev/null
+++ b/Source/core/css/pluginPlaceholder.css
@@ -0,0 +1,22 @@
+/*
+ * The style sheet loaded for plugin placeholders.

[haraken, 2014/09/01 00:59:43]

This comment should be written after the copyright. The copyright should be put
the first without any modification.

[jbroman, 2014/09/02 15:44:04]

This comment is removed in the latest patchset.

(I'd been following some of the older .css files we have, which have a
description ahead of the copyright header.)

+ *
+ * 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;
+}