content: Add an overridable task runner to ChildThreadImpl

content/public/child/child_thread.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #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"
 
 #if defined(OS_WIN)
 #include <windows.h>
 #endif
 
+namespace base {
+class SingleThreadTaskRunner;
+}
+
 namespace content {
 
 class CONTENT_EXPORT ChildThread : public IPC::Sender {
  public:
   // Returns the one child thread for this process.  Note that this can only be
   // accessed when running on the child thread itself.
   static ChildThread* Get();
 
   ~ChildThread() override {}
 
 #if defined(OS_WIN)
   // Request that the given font be loaded by the browser so it's cached by the
   // OS. Please see ChildProcessHost::PreCacheFont for details.
   virtual void PreCacheFont(const LOGFONT& log_font) = 0;
 
   // Release cached font.
   virtual void ReleaseCachedFonts() = 0;
 #endif
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_CHILD_CHILD_THREAD_H_