Use process/control messages (instead of RVH messages) for layout tests.

content/shell/app/shell_main_delegate.h

Index: content/shell/app/shell_main_delegate.h
diff --git a/content/shell/app/shell_main_delegate.h b/content/shell/app/shell_main_delegate.h
index 1297ff867b8c90c84a0d77506e6334946664c0c2..7bf71b31f762e7428066229199bf15b332e2f9e7 100644
--- a/content/shell/app/shell_main_delegate.h
+++ b/content/shell/app/shell_main_delegate.h
@@ -11,6 +11,7 @@
 #include "base/macros.h"
 #include "build/build_config.h"
 #include "content/public/app/content_main_delegate.h"
+#include "content/shell/common/shell_content_client.h"
 
 namespace content {
 class ContentClient;
@@ -45,7 +46,7 @@ class ShellMainDelegate : public ContentMainDelegate {
   std::unique_ptr<ShellContentBrowserClient> browser_client_;
   std::unique_ptr<ShellContentRendererClient> renderer_client_;
   std::unique_ptr<ShellContentUtilityClient> utility_client_;
-  std::unique_ptr<ContentClient> content_client_;
+  ShellContentClient content_client_;
 
 #if defined(OS_ANDROID)
   std::unique_ptr<BrowserMainRunner> browser_runner_;