Enable shadow DOM-based "missing plugin" placeholder.

content/public/renderer/content_renderer_client.cc

Index: content/public/renderer/content_renderer_client.cc
diff --git a/content/public/renderer/content_renderer_client.cc b/content/public/renderer/content_renderer_client.cc
index 76f27ee59e013128960b07c82102b1c7bd8b0f88..a1cb4c65296287ec57df45e3d07f6e6cba2f3867 100644
--- a/content/public/renderer/content_renderer_client.cc
+++ b/content/public/renderer/content_renderer_client.cc
@@ -14,6 +14,14 @@ SkBitmap* ContentRendererClient::GetSadWebViewBitmap() {
   return NULL;
 }
 
+blink::WebPluginPlaceholder*
+ContentRendererClient::OverrideCreatePluginPlaceholder(
+    RenderFrame* render_frame,
+    blink::WebLocalFrame* frame,
+    const blink::WebPluginParams& params) {
+  return nullptr;
+}
+
 bool ContentRendererClient::OverrideCreatePlugin(
     RenderFrame* render_frame,
     blink::WebLocalFrame* frame,