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

content/shell/browser/layout_test/layout_test_message_filter.cc

Index: content/shell/browser/layout_test/layout_test_message_filter.cc
diff --git a/content/shell/browser/layout_test/layout_test_message_filter.cc b/content/shell/browser/layout_test/layout_test_message_filter.cc
index 630417c49b6b0480da84eb3b1be1695dacf18793..470ba80e17a1fc47b6147fc92d5626e134698773 100644
--- a/content/shell/browser/layout_test/layout_test_message_filter.cc
+++ b/content/shell/browser/layout_test/layout_test_message_filter.cc
@@ -57,6 +57,7 @@ void LayoutTestMessageFilter::OverrideThreadForMessage(
     case LayoutTestHostMsg_ResetPermissions::ID:
     case LayoutTestHostMsg_LayoutTestRuntimeFlagsChanged::ID:
     case LayoutTestHostMsg_TestFinishedInSecondaryRenderer::ID:
+    case LayoutTestHostMsg_PrintMessage::ID:
       *thread = BrowserThread::UI;
       break;
   }
@@ -84,6 +85,8 @@ bool LayoutTestMessageFilter::OnMessageReceived(const IPC::Message& message) {
                         OnLayoutTestRuntimeFlagsChanged)
     IPC_MESSAGE_HANDLER(LayoutTestHostMsg_TestFinishedInSecondaryRenderer,
                         OnTestFinishedInSecondaryRenderer)
+    IPC_MESSAGE_HANDLER(LayoutTestHostMsg_PrintMessage,
+                        OnPrintMessage)
     IPC_MESSAGE_UNHANDLED(handled = false)
   IPC_END_MESSAGE_MAP()
 
@@ -201,4 +204,8 @@ void LayoutTestMessageFilter::OnTestFinishedInSecondaryRenderer() {
   BlinkTestController::Get()->OnTestFinishedInSecondaryRenderer();
 }
 
+void LayoutTestMessageFilter::OnPrintMessage(const std::string& message) {
+  BlinkTestController::Get()->OnPrintMessage(message);
+}
+
 }  // namespace content