Introduce intrinsic to expose debug state to generated code.

test/mjsunit/debug-is-active.js

+// Copyright 2014 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-debug-as debug --allow-natives-syntax
+
+Debug = debug.Debug;
+
+function f() { return %_DebugIsActive() != 0; }
+
+assertFalse(f());
+assertFalse(f());
+Debug.setListener(function() {});
+assertTrue(f());
+Debug.setListener(null);
+assertFalse(f());
+
+%OptimizeFunctionOnNextCall(f);
+assertFalse(f());
+assertOptimized(f);
+
+Debug.setListener(function() {});
+assertTrue(f());
+assertOptimized(f);
+
+Debug.setListener(null);
+assertFalse(f());
+assertOptimized(f);