Support sharing of ContentMain and BrowserMain code with embedded use cases.

content/shell/shell_browser_main.cc

Index: content/shell/shell_browser_main.cc
===================================================================
--- content/shell/shell_browser_main.cc	(revision 119665)
+++ content/shell/shell_browser_main.cc	(working copy)
@@ -1,4 +1,4 @@
-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Copyright (c) 2012 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.
 
@@ -46,6 +46,10 @@
 ShellBrowserMainParts::~ShellBrowserMainParts() {
 }
 
+MessageLoop* ShellBrowserMainParts::GetMainMessageLoop() {
+  return NULL;
+}
+
 void ShellBrowserMainParts::PreMainMessageLoopRun() {
   browser_context_.reset(new ShellBrowserContext(this));