content: Add an overridable task runner to ChildThreadImpl

content/public/child/child_thread.h

Index: content/public/child/child_thread.h
diff --git a/content/public/child/child_thread.h b/content/public/child/child_thread.h
index 6293ed164d8443dbd474ce35934a3235c0337943..5406c543dfb9531374c3ccbfd98fca792f4153de 100644
--- a/content/public/child/child_thread.h
+++ b/content/public/child/child_thread.h
@@ -5,6 +5,7 @@
 #ifndef CONTENT_PUBLIC_CHILD_CHILD_THREAD_H_
 #define CONTENT_PUBLIC_CHILD_CHILD_THREAD_H_
 
+#include "base/memory/ref_counted.h"
 #include "content/common/content_export.h"
 #include "ipc/ipc_sender.h"
 
@@ -12,6 +13,10 @@
 #include <windows.h>
 #endif
 
+namespace base {
+class SingleThreadTaskRunner;
+}
+
 namespace content {
 
 class CONTENT_EXPORT ChildThread : public IPC::Sender {