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

content/shell/renderer/test_runner/web_test_proxy.cc

Index: content/shell/renderer/test_runner/web_test_proxy.cc
diff --git a/content/shell/renderer/test_runner/web_test_proxy.cc b/content/shell/renderer/test_runner/web_test_proxy.cc
index bd31b74b1b469eea9912905f4fd2f9c28b6647a9..8fb3adb46a1d65ea993e48455d230093b33dc049 100644
--- a/content/shell/renderer/test_runner/web_test_proxy.cc
+++ b/content/shell/renderer/test_runner/web_test_proxy.cc
@@ -13,6 +13,7 @@
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/trace_event/trace_event.h"
+#include "components/plugins/renderer/test_plugin_placeholder.h"
 #include "content/shell/renderer/test_runner/accessibility_controller.h"
 #include "content/shell/renderer/test_runner/event_sender.h"
 #include "content/shell/renderer/test_runner/mock_color_chooser.h"
@@ -915,6 +916,9 @@ blink::WebPlugin* WebTestProxyBase::CreatePlugin(
     const blink::WebPluginParams& params) {
   if (TestPlugin::IsSupportedMimeType(params.mimeType))
     return TestPlugin::create(frame, params, delegate_);
+  if (params.mimeType == "application/x-plugin-placeholder-test")
+    return (new plugins::TestPluginPlaceholder(frame, params))->plugin();
+
   return 0;
 }