Add empty-handle checks to API functions (#ifdef ENABLE_EXTRA_CHECKS)

src/checks.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 266 matching lines @@
 #endif
 // Static asserts has no impact on runtime performance, so they can be
 // safely enabled in release mode. Moreover, the ((void) 0) expression
 // obeys different syntax rules than typedef's, e.g. it can't appear
 // inside class declaration, this leads to inconsistency between debug
 // and release compilation modes behavior.
 #define STATIC_ASSERT(test)  STATIC_CHECK(test)
 
 #define ASSERT_NOT_NULL(p)  ASSERT_NE(NULL, p)
 
+// "Extra checks" are lightweight checks that are enabled in some release
+// builds.
+#ifdef ENABLE_EXTRA_CHECKS
+#define EXTRA_CHECK(condition) CHECK(condition)
+#else
+#define EXTRA_CHECK(condition) ((void) 0)
+#endif
+
 #endif  // V8_CHECKS_H_