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

Source/core/html/shadow/PluginPlaceholderElement.idl

 // 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.
 
 [
     NoInterfaceObject
 ] interface PluginPlaceholderElement : HTMLDivElement {
     [ImplementedInPrivateScript, OnlyExposedToPrivateScript] attribute DOMString message;
     [ImplementedInPrivateScript, OnlyExposedToPrivateScript] void createCallback();
+
+    // Exposes the content of PluginPlaceholderElement.css.
+    [OnlyExposedToPrivateScript] readonly attribute DOMString styleSource;

[abarth-chromium, 2014/09/02 22:34:34]

Please don't add this function.  Instead, private scripts should have a generic
ability to call Platform::current()->loadResource(...)

[jbroman, 2014/09/02 23:53:32]

There are about three ways I can think of here. What seems best?

1. Providing some binding exposed to private script. This seems simplest.
Something like this:

    partial interface Window {
        [OnlyExposedToPrivateScript] DOMString loadPrivateResource(DOMString
name);
    }

2. Having chrome provide a special scheme for this, like it has
chrome-extension:// and such already. Then we could just have an @import of
blink-resource://PluginPlaceholderElement.css or similar. If I understand the
architecture of this, I think it would go to the browser process as part of
this, but that may not be a big deal. This might be generally useful (for
loading images etc. as well), but it's more complicated. We'd also have to
decide whether it's okay for these to just always be loadable (e.g. I think
web-accessible extension resources always are, but I'm not sure), or whether we
need to confine their loading to private script only.

3. Same as 2, but implemented in Blink before crossing the embedder API
boundary. Seems pointless as we'd just cross the embedder API boundary anyhow.

 };