Add plugins::TestPluginPlaceholder class and allow its use in Blink layout tests.

content/shell/renderer/layout_test/test_plugin_placeholder.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+#ifndef CONTENT_SHELL_RENDERER_TEST_RUNNER_TEST_PLUGIN_PLACEHOLDER_H_
+#define CONTENT_SHELL_RENDERER_TEST_RUNNER_TEST_PLUGIN_PLACEHOLDER_H_
+
+#include "components/plugins/renderer/plugin_placeholder.h"
+
+namespace content {
+
+// A subclass of PluginPlaceholder for use in Blink layout tests.
+class TestPluginPlaceholder : public plugins::PluginPlaceholder {
+ public:
+  TestPluginPlaceholder(blink::WebLocalFrame* frame,
+                        const blink::WebPluginParams& params);
+
+  void BindWebFrame(blink::WebFrame* frame) override;
+};
+
+}  // namespace content
+
+#endif  // CONTENT_SHELL_RENDERER_TEST_RUNNER_TEST_PLUGIN_PLACEHOLDER_H_