Update comments on context slots.

src/contexts.h

Index: src/contexts.h
diff --git a/src/contexts.h b/src/contexts.h
index 2978012d17d36328346934d7ee2280e5744b668e..ae6629e35d50f1d688790157d02a58df378dfcc1 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -333,9 +333,7 @@ class ScriptContextTable : public FixedArray {
 //                statically allocated context slots. The names are needed
 //                for dynamic lookups in the presence of 'with' or 'eval'.
 //
-// [ previous  ]  A pointer to the previous context. It is NULL for
-//                function contexts, and non-NULL for 'with' contexts.
-//                Used to implement the 'with' statement.
+// [ previous  ]  A pointer to the previous context.
 //
 // [ extension ]  A pointer to an extension JSObject, or "the hole". Used to
 //                implement 'with' statements and dynamic declarations
@@ -350,9 +348,7 @@ class ScriptContextTable : public FixedArray {
 //                SloppyBlockWithEvalContextExtension, pairing the ScopeInfo
 //                with an extension object.
 //
-// [ global_object ]  A pointer to the global object. Provided for quick
-//                access to the global object from inside the code (since
-//                we always have a context pointer).
+// [ native_context ]  A pointer to the native context.
 //
 // In addition, function contexts may have statically allocated context slots
 // to store local variables/functions that are accessed from inner functions