Enable "strict mode"; for debugger scripts

BUG=v8:3708

Review URL: https://codereview.chromium.org/736583007

Cr-Commit-Position: refs/heads/master@{#25420}

src/debug-debugger.js

 // 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 = [];

src/mirror-debugger.js

 // Copyright 2006-2012 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";
 
 // Handle id counters.
 var next_handle_ = 0;
@@ -44,7 +45,7 @@ function MakeMirror(value, opt_transient) {
 
   // Look for non transient mirrors in the mirror cache.
   if (!opt_transient && mirror_cache_enabled_) {
-    for (id in mirror_cache_) {
+    for (var id in mirror_cache_) {
       mirror = mirror_cache_[id];
       if (mirror.value() === value) {
         return mirror;
@@ -1287,11 +1288,11 @@ ErrorMirror.prototype.toText = function() {
   // Use the same text representation as in messages.js.
   var text;
   try {
-    str = %_CallFunction(this.value_, builtins.ErrorToString);
+    text = %_CallFunction(this.value_, builtins.ErrorToString);
   } catch (e) {
-    str = '#<Error>';
+    text = '#<Error>';
   }
-  return str;
+  return text;
 };