Don't create debug context if debug listener is not set

test/mjsunit/regress/regress-3960.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: --allow-natives-syntax
+// Flags: --allow-natives-syntax --expose-debug-as debug

[Yang, 2015/05/11 06:44:19]

This makes this test pointless (see comment). Since %GetDebugContext is only
used in tests, please change it to set a dummy listener to activate the debugger
before getting the debug context (and reset it after).

[yurys, 2015/05/12 06:46:59]

Done. I added DebugScope to Runtime_GetDebugContext.

This sounds weird to me that we allow setting breakpoints when there is no debug
listener. I'd rather we cleared all breakpoints on removing debug listener and
require the client to restore all breakpoints at the beginning of the next debug
session. With current behavior we can set breakpoints during the first debug
session and then hit them in the second session without even knowing that there
are breakpoints. I vaguely recall a bug on this in WebKit but I cannot find it
at the moment.

 
-// Test that setting break point is works correctly when the debugger is
+// Test that setting break point works correctly when the debugger is
 // activated late, which leads to duplicate shared function infos.
 
 (function() {
-  var Debug = %GetDebugContext().Debug;
+  var Debug = debug.Debug;
 
   function listener(event, exec_state, event_data, data) {
     if (event != Debug.DebugEvent.Break) return;
     try {
       assertTrue(/foo/.test(exec_state.frame(0).sourceLineText()));
       break_count++;
     } catch (e) {
       exception = e;
     }
   }
 
   for (var i = 0; i < 3; i++) {
     var foo = function() { a = 1; }
     var exception = null;
     var break_count = 0;
     Debug.setListener(listener);
     if (i < 2) Debug.setBreakPoint(foo, 0, 0);
     assertTrue(/\[B\d\]a = 1/.test(Debug.showBreakPoints(foo)));
     foo();
     assertEquals(1, break_count);
     assertNull(exception);
   }
 
   Debug.setListener(null);
 })();