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

content/shell/renderer/layout_test/blink_test_runner.h

 // Copyright (c) 2012 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_LAYOUT_TEST_BLINK_TEST_RUNNER_H_
 #define CONTENT_SHELL_RENDERER_LAYOUT_TEST_BLINK_TEST_RUNNER_H_
 
 #include <vector>
 
 #include "base/files/file_path.h"
@@ skipping 113 matching lines @@
   blink::WebLayer* InstantiateWebLayer(
       scoped_refptr<cc::TextureLayer> layer) override;
   cc::SharedBitmapManager* GetSharedBitmapManager() override;
   void DispatchBeforeInstallPromptEvent(
       int request_id,
       const std::vector<std::string>& event_platforms,
       const base::Callback<void(bool)>& callback) override;
   void ResolveBeforeInstallPromptPromise(
       int request_id,
       const std::string& platform) override;
+  blink::WebPlugin* CreatePluginPlaceholder(
+    blink::WebLocalFrame* frame,
+    const blink::WebPluginParams& params) override;
 
   void Reset();
 
   void set_proxy(WebTestProxyBase* proxy) { proxy_ = proxy; }
   WebTestProxyBase* proxy() const { return proxy_; }
 
   void ReportLeakDetectionResult(const LeakDetectionResult& result);
 
  private:
   // Message handlers.
@@ skipping 30 matching lines @@
 
   scoped_ptr<LeakDetector> leak_detector_;
   bool needs_leak_detector_;
 
   DISALLOW_COPY_AND_ASSIGN(BlinkTestRunner);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_SHELL_RENDERER_LAYOUT_TEST_BLINK_TEST_RUNNER_H_