Make BrowserMainParts have an empty implementation to make embedding easier.

content/shell/shell_browser_main_parts.h

Index: content/shell/shell_browser_main_parts.h
===================================================================
--- content/shell/shell_browser_main_parts.h	(revision 139582)
+++ content/shell/shell_browser_main_parts.h	(working copy)
@@ -27,15 +27,9 @@
 
   // content::BrowserMainParts overrides.
   virtual void PreEarlyInitialization() OVERRIDE;
-  virtual void PostEarlyInitialization() OVERRIDE {}
   virtual void PreMainMessageLoopStart() OVERRIDE;
-  virtual void PostMainMessageLoopStart() OVERRIDE {}
-  virtual void ToolkitInitialized() OVERRIDE {}
-  virtual int PreCreateThreads() OVERRIDE;
   virtual void PreMainMessageLoopRun() OVERRIDE;
-  virtual bool MainMessageLoopRun(int* result_code) OVERRIDE;
   virtual void PostMainMessageLoopRun() OVERRIDE;
-  virtual void PostDestroyThreads() OVERRIDE {}
 
   ShellDevToolsDelegate* devtools_delegate() { return devtools_delegate_; }