Implement support for closing shadow DOM plugin placeholders.

public/web/WebPluginPlaceholder.h

Index: public/web/WebPluginPlaceholder.h
diff --git a/public/web/WebPluginPlaceholder.h b/public/web/WebPluginPlaceholder.h
index 0996c36991d42ee029e563a190d6857bf26f891b..e0400b5a2dfcdf65ced94dd97ab7c10818b1456e 100644
--- a/public/web/WebPluginPlaceholder.h
+++ b/public/web/WebPluginPlaceholder.h
@@ -16,6 +16,9 @@ public:
 
     // Returns a plaintext message to be displayed.
     virtual WebString message() const { return WebString(); }
+
+    // If set, the placeholder will present UI to permit dismissing it.
+    virtual bool isCloseable() const { return false; }
 };
 
 } // namespace blink