Mojo: Split public/cpp/system/core.h into pieces

mojo/public/cpp/system/macros.h

Index: mojo/public/cpp/system/macros.h
diff --git a/mojo/public/cpp/system/macros.h b/mojo/public/cpp/system/macros.h
index ac4f3dbc84cd293896b1d7e784bbee562cce1ba0..feb352d8de1b56494ec577fabe322a5cb2743062 100644
--- a/mojo/public/cpp/system/macros.h
+++ b/mojo/public/cpp/system/macros.h
@@ -7,6 +7,10 @@
 
 #include "mojo/public/c/system/macros.h"
 
+// Define a set of C++ specific macros.
+// Mojo C++ API users can assume that mojo/public/cpp/system/macros.h
+// includes mojo/public/c/system/macros.h.
+
 // Annotate a virtual method indicating it must be overriding a virtual method
 // in the parent class. Use like:
 //   virtual void foo() OVERRIDE;