Enable "strict mode"; for debugger scripts

src/debug-debugger.js

Index: src/debug-debugger.js
diff --git a/src/debug-debugger.js b/src/debug-debugger.js
index 9be2b044d31647b30f04c164d7f83c487e9a3145..09f479e1e8b6814aca5927c1f14ca353f20edf62 100644
--- a/src/debug-debugger.js
+++ b/src/debug-debugger.js
@@ -1,6 +1,7 @@
 // Copyright 2012 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.
+"use strict";
 
 // Default number of frames to include in the response to backtrace request.
 var kDefaultBacktraceLength = 10;
@@ -672,7 +673,7 @@ Debug.setBreakPointByScriptIdAndPosition = function(script_id, position,
                                                     condition, enabled,
                                                     opt_position_alignment)
 {
-  break_point = MakeBreakPoint(position);
+  var break_point = MakeBreakPoint(position);
   break_point.setCondition(condition);
   if (!enabled) {
     break_point.disable();
@@ -739,7 +740,7 @@ Debug.clearBreakPoint = function(break_point_number) {
 
 Debug.clearAllBreakPoints = function() {
   for (var i = 0; i < break_points.length; i++) {
-    break_point = break_points[i];
+    var break_point = break_points[i];
     %ClearBreakPoint(break_point);
   }
   break_points = [];