Adds the ability for MessageLoop to take a MessagePump

base/message_loop/message_loop.h

Index: base/message_loop/message_loop.h
diff --git a/base/message_loop/message_loop.h b/base/message_loop/message_loop.h
index 0eb180352d324eb1f9a3dbff6dbe58eaa44e1bf2..30570725c55934e7aec2f644b6b793a71e04064b 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -140,9 +140,13 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
 #endif // defined(OS_ANDROID)
   };
 
+  // Creates a MessageLoop. If |message_pump| is NULL a MessagePump based on
+  // |type| is used, otherwise MessageLoop takes ownership of |message_pump| and
+  // uses it as the MessagePump.
   // Normally, it is not necessary to instantiate a MessageLoop.  Instead, it
   // is typical to make use of the current thread's MessageLoop instance.
-  explicit MessageLoop(Type type = TYPE_DEFAULT);
+  // TODO(sky): remove default args.
+  MessageLoop(Type type = TYPE_DEFAULT, MessagePump* message_pump = NULL);

[darin (slow to review), 2013/11/07 00:17:54]

Can we use scoped_ptr<MessagePump> here? Is it hard to express a null default
value in that case?

Another option would be to just override the constructor entirely.  I suspect
the Type field is not meaningful when initializing a MessageLoop with a custom
MessagePump. Internally, we should probably have a MessageLoop type value of
TYPE_CUSTOM.

   virtual ~MessageLoop();
 
   // Returns the MessageLoop object for the current thread, or null if none.