Enable shadow DOM-based "missing plugin" placeholder.

This is guarded by a command-line switch.

TEST=OverrideCreatePluginTest
BUG=364716

Review URL: https://codereview.chromium.org/649873006

Cr-Commit-Position: refs/heads/master@{#301996}
diff --git a/content/renderer/render_frame_impl.h b/content/renderer/render_frame_impl.h
index dad7b18..b10ab7d 100644
--- a/content/renderer/render_frame_impl.h
+++ b/content/renderer/render_frame_impl.h
@@ -282,6 +282,9 @@
                                       v8::Handle<v8::Context> context) override;
 
   // blink::WebFrameClient implementation:
+  blink::WebPluginPlaceholder* createPluginPlaceholder(
+      blink::WebLocalFrame*,
+      const blink::WebPluginParams&) override;
   virtual blink::WebPlugin* createPlugin(blink::WebLocalFrame* frame,
                                          const blink::WebPluginParams& params);
   // TODO(jrummell): Remove this method once blink updated.