Enable shadow DOM-based "missing plugin" placeholder.

content/renderer/render_frame_impl.cc

Index: content/renderer/render_frame_impl.cc
diff --git a/content/renderer/render_frame_impl.cc b/content/renderer/render_frame_impl.cc
index 0ca01ad816c2115d2f4f59d147d63c04bac880dc..83230a16ede6e70ea947083c0b93ef1c2ed5f8ef 100644
--- a/content/renderer/render_frame_impl.cc
+++ b/content/renderer/render_frame_impl.cc
@@ -1605,6 +1605,14 @@ void RenderFrameImpl::EnsureMojoBuiltinsAreAvailable(
 
 // blink::WebFrameClient implementation ----------------------------------------
 
+blink::WebPluginPlaceholder* RenderFrameImpl::createPluginPlaceholder(
+    blink::WebLocalFrame* frame,
+    const blink::WebPluginParams& params) {
+  DCHECK_EQ(frame_, frame);
+  return GetContentClient()->renderer()->OverrideCreatePluginPlaceholder(
+      this, frame, params);
+}
+
 blink::WebPlugin* RenderFrameImpl::createPlugin(
     blink::WebLocalFrame* frame,
     const blink::WebPluginParams& params) {