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

content/shell/renderer/layout_test/blink_test_runner.cc

Index: content/shell/renderer/layout_test/blink_test_runner.cc
diff --git a/content/shell/renderer/layout_test/blink_test_runner.cc b/content/shell/renderer/layout_test/blink_test_runner.cc
index a69c2fff5bf3290e0601b357b3ffc56f378d3a63..64ee214d417e85650ed1b9f36d41e8c549012934 100644
--- a/content/shell/renderer/layout_test/blink_test_runner.cc
+++ b/content/shell/renderer/layout_test/blink_test_runner.cc
@@ -36,6 +36,7 @@
 #include "content/shell/renderer/layout_test/gc_controller.h"
 #include "content/shell/renderer/layout_test/layout_test_render_process_observer.h"
 #include "content/shell/renderer/layout_test/leak_detector.h"
+#include "content/shell/renderer/layout_test/test_plugin_placeholder.h"
 #include "content/shell/renderer/test_runner/app_banner_client.h"
 #include "content/shell/renderer/test_runner/gamepad_controller.h"
 #include "content/shell/renderer/test_runner/mock_screen_orientation_client.h"
@@ -672,7 +673,7 @@ void BlinkTestRunner::DispatchBeforeInstallPromptEvent(
   callback.Run(reply == blink::WebAppBannerPromptReply::Cancel);
 }
 
-  void BlinkTestRunner::ResolveBeforeInstallPromptPromise(
+void BlinkTestRunner::ResolveBeforeInstallPromptPromise(
       int request_id, const std::string& platform) {
   WebTestInterfaces* interfaces =
       LayoutTestRenderProcessObserver::GetInstance()->test_interfaces();
@@ -680,6 +681,13 @@ void BlinkTestRunner::DispatchBeforeInstallPromptEvent(
       request_id, platform);
 }
 
+blink::WebPlugin* BlinkTestRunner::CreatePluginPlaceholder(
+    blink::WebLocalFrame* frame, const blink::WebPluginParams& params) {
+  if (params.mimeType == "application/x-plugin-placeholder-test")
+    return (new TestPluginPlaceholder(frame, params))->plugin();
+  return 0;
+}
+
 // RenderViewObserver  --------------------------------------------------------
 
 void BlinkTestRunner::DidClearWindowObject(WebLocalFrame* frame) {