Enable shadow DOM-based "missing plugin" placeholder.

content/public/renderer/content_renderer_client.h

Index: content/public/renderer/content_renderer_client.h
diff --git a/content/public/renderer/content_renderer_client.h b/content/public/renderer/content_renderer_client.h
index daf8d4a650d7212d3725d12dc7160a71f17cc332..53864f715a77a6bad7486614dee54cd25095af58 100644
--- a/content/public/renderer/content_renderer_client.h
+++ b/content/public/renderer/content_renderer_client.h
@@ -37,6 +37,7 @@ class WebMediaStreamCenter;
 class WebMediaStreamCenterClient;
 class WebPlugin;
 class WebPluginContainer;
+class WebPluginPlaceholder;
 class WebPrescientNetworking;
 class WebRTCPeerConnectionHandler;
 class WebRTCPeerConnectionHandlerClient;
@@ -87,6 +88,14 @@ class CONTENT_EXPORT ContentRendererClient {
   // none.
   virtual SkBitmap* GetSadWebViewBitmap();
 
+  // Allows the embedder to create a plugin placeholder instead of a plugin.
+  // Called before OverrideCreatePlugin. May return null to decline to provide
+  // a plugin placeholder.
+  virtual blink::WebPluginPlaceholder* OverrideCreatePluginPlaceholder(
+      RenderFrame* render_frame,
+      blink::WebLocalFrame* frame,
+      const blink::WebPluginParams& params);
+
   // Allows the embedder to override creating a plugin. If it returns true, then
   // |plugin| will contain the created plugin, although it could be NULL. If it
   // returns false, the content layer will create the plugin.