Debugger: require debugger to be active when dealing with breaks.

test/mjsunit/regress/regress-crbug-424142.js

Index: test/mjsunit/regress/regress-crbug-424142.js
diff --git a/test/mjsunit/regress/regress-crbug-424142.js b/test/mjsunit/regress/regress-crbug-424142.js
index 0a370d414269062ff9dabae4e09e4e7a2e77ea13..b188565fe2f22ac79dcf3ea374e222fb61622d88 100644
--- a/test/mjsunit/regress/regress-crbug-424142.js
+++ b/test/mjsunit/regress/regress-crbug-424142.js
@@ -23,6 +23,7 @@
 function sentinel() {}
 
 Debug = debug.Debug;
+Debug.setListener(function(){});
 
 var script = Debug.findScript(sentinel);
 var line = 14;
@@ -34,3 +35,5 @@ var actual = Debug.setBreakPointByScriptIdAndPosition(
 // the break point.
 assertTrue(line_start <= actual);
 assertTrue(actual <= line_end);
+
+Debug.setListener(null);