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

content/shell/renderer/test_runner/web_test_proxy.cc

 // Copyright 2014 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/web_test_proxy.h"
 
 #include <cctype>
 
 #include "base/callback_helpers.h"
 #include "base/command_line.h"
@@ skipping 897 matching lines @@
     delegate_->PrintMessage(std::string("createView(") +
                             URLDescription(request.url()) + ")\n");
   return true;
 }
 
 blink::WebPlugin* WebTestProxyBase::CreatePlugin(
     blink::WebLocalFrame* frame,
     const blink::WebPluginParams& params) {
   if (TestPlugin::IsSupportedMimeType(params.mimeType))
     return TestPlugin::create(frame, params, delegate_);
-  return 0;
+  return delegate_->CreatePluginPlaceholder(frame, params);
 }
 
 void WebTestProxyBase::SetStatusText(const blink::WebString& text) {
   if (!test_interfaces_->GetTestRunner()->shouldDumpStatusCallbacks())
     return;
   delegate_->PrintMessage(
       std::string("UI DELEGATE STATUS CALLBACK: setStatusText:") +
       text.utf8().data() + "\n");
 }
 
@@ skipping 464 matching lines @@
   // to cancel the input method's ongoing composition session.
   if (web_widget_)
     web_widget_->confirmComposition();
 }
 
 blink::WebString WebTestProxyBase::acceptLanguages() {
   return blink::WebString::fromUTF8(accept_languages_);
 }
 
 }  // namespace content