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 27abd2b2867627b8072093678f39a86ac527dd28..1fb84cd166d3c5d40011029459322e2cbd529127 100644
--- a/content/shell/renderer/test_runner/web_test_proxy.cc
+++ b/content/shell/renderer/test_runner/web_test_proxy.cc
@@ -23,6 +23,7 @@
 #include "content/shell/renderer/test_runner/spell_check_client.h"
 #include "content/shell/renderer/test_runner/test_interfaces.h"
 #include "content/shell/renderer/test_runner/test_plugin.h"
+#include "content/shell/renderer/test_runner/test_plugin_placeholder.h"
 #include "content/shell/renderer/test_runner/test_runner.h"
 #include "content/shell/renderer/test_runner/web_test_delegate.h"
 #include "content/shell/renderer/test_runner/web_test_interfaces.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 TestPluginPlaceholder(frame, params))->plugin();
+
   return 0;
 }