Do not clear stepping after DebugEvaluate.

test/mjsunit/regress/regress-crbug-467180.js

+// Copyright 2015 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
+
+function f() {
+  for (var i = 10; i < 14; i++) {  // 1
+    i;                             // 2
+  }
+}                                  // 3
+
+var state = "conditional";
+var log = [];
+var exception = null;
+
+function listener(event, exec_state, event_data, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    var label = +exec_state.frame(0).sourceLineText().substr(-1);
+    log.push(label);
+    if (label == 2) log.push(exec_state.frame(0).evaluate("i").value());
+    exec_state.prepareStep(Debug.StepAction.StepNext, 1);
+  } catch (e) {
+    exception = e;
+    print("Caught something. " + e + " " + e.stack);
+  };
+};
+
+
+var Debug = debug.Debug;
+Debug.setListener(listener);
+
+Debug.setBreakPoint(f, 2, 0, "i == 12");
+
+f();
+
+Debug.setListener(null);  // 4
+
+assertEquals([2,12,1,1,2,13,1,1,3,4], log);
+assertNull(exception);