Set up content in-process main threads via factory

content/public/browser/render_process_host.h

Index: content/public/browser/render_process_host.h
diff --git a/content/public/browser/render_process_host.h b/content/public/browser/render_process_host.h
index ccc1e72e536cc5aed7cf48782be171b35a89ce2f..35627c78b63c29a0ccb3935c23c8cd41f8ec154c 100644
--- a/content/public/browser/render_process_host.h
+++ b/content/public/browser/render_process_host.h
@@ -30,6 +30,9 @@ class TimeDelta;
 
 namespace content {
 
+typedef base::Thread* (*RendererMainThreadFactoryFunction)(
+    const std::string& id);
+
 // Interface that represents the browser side of the browser <-> renderer
 // communication channel. There will generally be one RenderProcessHost per
 // renderer process.
@@ -248,6 +251,9 @@ class CONTENT_EXPORT RenderProcessHost : public IPC::Sender,
   // Returns the current max number of renderer processes used by the content
   // module.
   static size_t GetMaxRendererProcessCount();
+
+  static void RegisterRendererMainThreadFactory(
+      RendererMainThreadFactoryFunction create);
 };
 
 }  // namespace content.