Allow HTMLPlugInElement to host UA shadow DOM.

Source/core/testing/Internals.cpp

Index: Source/core/testing/Internals.cpp
diff --git a/Source/core/testing/Internals.cpp b/Source/core/testing/Internals.cpp
index 837b199d1ee7865d9a6cdeeac4b9e23cea8f2cd7..1eeeb991af9c0813ae1dda1a6cddd7897b0e45fd 100644
--- a/Source/core/testing/Internals.cpp
+++ b/Source/core/testing/Internals.cpp
@@ -77,6 +77,7 @@
 #include "core/html/HTMLIFrameElement.h"
 #include "core/html/HTMLInputElement.h"
 #include "core/html/HTMLMediaElement.h"
+#include "core/html/HTMLPlugInElement.h"
 #include "core/html/HTMLSelectElement.h"
 #include "core/html/HTMLTextAreaElement.h"
 #include "core/html/canvas/CanvasRenderingContext2D.h"
@@ -2226,4 +2227,18 @@ void Internals::hideAllTransitionElements()
         frame()->document()->hideTransitionElements(AtomicString(iter->selector));
 }
 
+void Internals::forcePluginPlaceholder(HTMLElement* element, const String& htmlSource, ExceptionState& exceptionState)
+{
+    if (!element) {
+        exceptionState.throwDOMException(InvalidAccessError, ExceptionMessages::argumentNullOrIncorrectType(1, "HTMLElement"));
+        return;
+    }
+    if (!element->isPluginElement()) {
+        exceptionState.throwDOMException(InvalidNodeTypeError, "The element provided is not a plugin.");
+        return;
+    }
+    element->ensureUserAgentShadowRoot().setInnerHTML(htmlSource, exceptionState);
+    toHTMLPlugInElement(element)->setUsePlaceholderContent(true);
+}
+
 } // namespace blink