Have content/ create and destroy its own threads.

chrome/browser/io_thread.h

Index: chrome/browser/io_thread.h
diff --git a/chrome/browser/io_thread.h b/chrome/browser/io_thread.h
index da098396ff7a49838c168e76261d3e7d69055cd8..19e6f5594391125ce0969616e1589129909e313d 100644
--- a/chrome/browser/io_thread.h
+++ b/chrome/browser/io_thread.h
@@ -9,12 +9,13 @@
 #include <string>
 
 #include "base/basictypes.h"
+#include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/task.h"
 #include "chrome/browser/net/ssl_config_service_manager.h"
 #include "chrome/browser/prefs/pref_member.h"
-#include "content/browser/browser_process_sub_thread.h"
+#include "content/public/browser/io_thread_delegate.h"
 #include "net/base/network_change_notifier.h"
 
 class ChromeNetLog;
@@ -49,7 +50,10 @@ class URLRequestContextGetter;
 class URLSecurityManager;
 }  // namespace net
 
-class IOThread : public content::BrowserProcessSubThread {
+// Contains state associated with, initialized and cleaned up on, and
+// primarily used on, the IO thread.  Also acts as a convenience
+// accessor to the Thread object for the IO thread.
+class IOThread : public content::IOThreadDelegate {
  public:
   struct Globals {
     Globals();
@@ -115,9 +119,14 @@ class IOThread : public content::BrowserProcessSubThread {
   // called on the IO thread.
   void ClearHostCache();
 
+  // Convenience method similar to base::Thread, giving access to the
+  // actual IO thread.
+  // TODO(joi): Remove this in follow-up changes.
+  MessageLoop* message_loop() const;
+
  protected:
-  virtual void Init();
-  virtual void CleanUp();
+  virtual void Init() OVERRIDE;
+  virtual void CleanUp() OVERRIDE;
 
  private:
   // Provide SystemURLRequestContextGetter with access to