More header cleanup:

base/message_loop.h

Index: base/message_loop.h
diff --git a/base/message_loop.h b/base/message_loop.h
index 8deacecd10d293a857ed736809549dee8baa5903..35b2651d85b0d1d9bcbffac0b6b0109dfe027d02 100644
--- a/base/message_loop.h
+++ b/base/message_loop.h
@@ -9,11 +9,10 @@
 #include <string>
 
 #include "base/basictypes.h"
-#include "base/histogram.h"
+#include "base/lock.h"
 #include "base/message_pump.h"
 #include "base/observer_list.h"
 #include "base/ref_counted.h"
-#include "base/scoped_ptr.h"
 #include "base/task.h"
 
 #if defined(OS_WIN)
@@ -27,6 +26,8 @@
 #endif
 #endif
 
+class Histogram;
+
 // A MessageLoop is used to process events for a particular thread.  There is
 // at most one MessageLoop instance per thread.
 //
@@ -427,9 +428,6 @@ class MessageLoop : public base::MessagePump::Delegate {
   // If message_histogram_ is NULL, this is a no-op.
   void HistogramEvent(int event);
 
-  static const LinearHistogram::DescriptionPair event_descriptions_[];
-  static bool enable_histogrammer_;
-
   Type type_;
 
   // A list of tasks that need to be processed by this instance.  Note that