Introduce object visitor to estimate the size of a native context.

test/cctest/test-api.cc

 // 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 21809 matching lines @@
 
   v8::TryCatch try_catch(CcTest::isolate());
   timeout_thread.Start();
 
   CompileRun(
       "var ab = new SharedArrayBuffer(4);"
       "var i32a = new Int32Array(ab);"
       "Atomics.futexWait(i32a, 0, 0);");
   CHECK(try_catch.HasTerminated());
 }
+
+
+TEST(EstimatedContextSize) {
+  v8::Isolate* isolate = CcTest::isolate();
+  v8::HandleScope scope(isolate);
+  LocalContext env;
+  CHECK(50000 < env->EstimatedSize());
+}