[interpreter, debugger] implement debugger statement.

test/mjsunit/ignition/debugger-statement.js

+// Copyright 2016 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: --ignition-filter=f --expose-debug-as debug
+
+var Debug = debug.Debug;
+
+var break_count = 0;
+
+function f() {
+  debugger;
+}
+
+function listener(event, exec_data) {
+  if (event != Debug.DebugEvent.Break) return;
+  break_count++;
+}
+
+f();
+assertEquals(0, break_count);
+
+Debug.setListener(listener);
+
+f();
+assertEquals(1, break_count);
+
+Debug.setListener(null);
+
+f();
+assertEquals(1, break_count);