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

content/shell/renderer/layout_test/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/layout_test/test_plugin_placeholder.h"
+
+#include "gin/handle.h"
+#include "third_party/WebKit/public/web/WebKit.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")) {
+}
+
+void TestPluginPlaceholder::BindWebFrame(blink::WebFrame* frame) {
+  v8::Isolate* isolate = blink::mainThreadIsolate();
+  v8::HandleScope handle_scope(isolate);
+  v8::Local<v8::Context> context = frame->mainWorldScriptContext();
+  DCHECK(!context.IsEmpty());
+
+  v8::Context::Scope context_scope(context);
+  v8::Local<v8::Object> global = context->Global();
+  global->Set(gin::StringToV8(isolate, "plugin"),
+              gin::CreateHandle(isolate, this).ToV8());
+}
+
+}  // namespace content