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

content/shell/renderer/test_runner/test_plugin_placeholder.cc

+// 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.
+
+#include "content/shell/renderer/test_runner/test_plugin_placeholder.h"
+
+namespace content {
+
+TestPluginPlaceholder::TestPluginPlaceholder(
+    blink::WebLocalFrame* frame,
+    const blink::WebPluginParams& params)
+    : PluginPlaceholder(nullptr,
+                        frame,
+                        params,
+                        "<div>Test content</div>",
+                        GURL("http://www.test.com")) {
+  LOG(ERROR) << "TestPluginPlaceholder::TestPluginPlaceholder";

[tommycli, 2015/05/28 21:13:33]

Stray log message.

[chrishtr, 2015/05/28 21:16:52]

Fixed.

+}
+
+}  // namespace content