Explicitly initialize secondary renderers for layout tests.

content/shell/common/shell_messages.h

Index: content/shell/common/shell_messages.h
diff --git a/content/shell/common/shell_messages.h b/content/shell/common/shell_messages.h
index f9146b99cc917fe9a34a0ecbe00d5a09507ec291..c978e7170f2b3021f1c8b911f9992dd90dbcf4f6 100644
--- a/content/shell/common/shell_messages.h
+++ b/content/shell/common/shell_messages.h
@@ -36,16 +36,22 @@ IPC_MESSAGE_ROUTED0(ShellViewMsg_Reset)
 IPC_MESSAGE_CONTROL1(ShellViewMsg_SetWebKitSourceDir,
                      base::FilePath /* webkit source dir */)
 
-// Sets the test config for a layout test that is being started.
+// Sets the test config for a layout test that is being started.  This message
+// is sent only to a renderer that hosts parts of the main test window.
 IPC_MESSAGE_ROUTED1(ShellViewMsg_SetTestConfiguration,
                     content::ShellTestConfiguration)
 
-// Replicates test config (for an already started test) to a new renderer.
+// Replicates test config (for an already started test) to a new renderer
+// that hosts parts of the main test window.
 IPC_MESSAGE_ROUTED2(
     ShellViewMsg_ReplicateTestConfiguration,
     content::ShellTestConfiguration,
     base::DictionaryValue /* accumulated_layout_test_runtime_flags_changes */)
 
+// Sets up a secondary renderer (renderer that doesn't [yet] host parts of the
+// main test window) for a layout test.
+IPC_MESSAGE_ROUTED0(ShellViewMsg_SetupSecondaryRenderer)
+
 // Used to broadcast changes happening in one renderer to all other renderers.
 IPC_MESSAGE_ROUTED1(
     ShellViewMsg_ReplicateLayoutTestRuntimeFlagsChanges,