content: Add an overridable task runner to ChildThreadImpl

content/child/child_thread_impl.h

Index: content/child/child_thread_impl.h
diff --git a/content/child/child_thread_impl.h b/content/child/child_thread_impl.h
index 03727d6b0477ef8fc529230e2c763b9d5e0a01b9..87a8e6ba8971dd1691c566ee93b1ee36af305b97 100644
--- a/content/child/child_thread_impl.h
+++ b/content/child/child_thread_impl.h
@@ -81,6 +81,12 @@ class CONTENT_EXPORT ChildThreadImpl
   bool Send(IPC::Message* msg) override;
 
   // ChildThread implementation:
+  // Returns the task runner which should be used to post tasks to run on this
+  // child thread. Note that this may be different from the underlying message
+  // loop returned by message_loop(). For this reason, only use message_loop()
+  // if you need to directly manipulate the underlying message loop, e.g., by
+  // adding a task listener instead of just posting tasks for execution.
+  virtual scoped_refptr<base::SingleThreadTaskRunner> GetTaskRunner();
 #if defined(OS_WIN)
   void PreCacheFont(const LOGFONT& log_font) override;
   void ReleaseCachedFonts() override;
@@ -166,6 +172,7 @@ class CONTENT_EXPORT ChildThreadImpl
     return resource_message_filter_.get();
   }
 
+  // See note in GetTaskRunner().
   base::MessageLoop* message_loop() const { return message_loop_; }
 
   // Returns the one child thread. Can only be called on the main thread.
@@ -263,6 +270,7 @@ class CONTENT_EXPORT ChildThreadImpl
   bool on_channel_error_called_;
 
   base::MessageLoop* message_loop_;
+  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
 
   scoped_ptr<FileSystemDispatcher> file_system_dispatcher_;
 
@@ -314,6 +322,7 @@ struct ChildThreadImpl::Options {
   bool use_mojo_channel;
   bool in_browser_process;
   std::vector<IPC::MessageFilter*> startup_filters;
+  scoped_refptr<base::SingleThreadTaskRunner> task_runner;
 
  private:
   Options();
@@ -327,6 +336,8 @@ class ChildThreadImpl::Options::Builder {
   Builder& UseMojoChannel(bool use_mojo_channel);
   Builder& WithChannelName(const std::string& channel_name);
   Builder& AddStartupFilter(IPC::MessageFilter* filter);
+  Builder& WithTaskRunner(
+      scoped_refptr<base::SingleThreadTaskRunner> task_runner);
 
   Options Build();