Group lexical context variables for faster look up.

src/contexts.h

Index: src/contexts.h
diff --git a/src/contexts.h b/src/contexts.h
index 9c8dc8a860ab2e86cca55933a9aec93bc5e0f83a..d5b090618f13e06d92070518e743513352363673 100644
--- a/src/contexts.h
+++ b/src/contexts.h
@@ -244,6 +244,10 @@ class ScriptContextTable : public FixedArray {
                      LookupResult* result);
 
   MUST_USE_RESULT
+  static bool LookupLexical(Handle<ScriptContextTable> table,
+                            Handle<String> name);
+
+  MUST_USE_RESULT
   static Handle<ScriptContextTable> Extend(Handle<ScriptContextTable> table,
                                            Handle<Context> script_context);