Fix the way sourceRoot is resolved in source maps.

Source/devtools/front_end/SourceMap.js

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 153 matching lines @@
     {
         var sourceIndex = 0;
         var sourceLineNumber = 0;
         var sourceColumnNumber = 0;
         var nameIndex = 0;
 
         var sources = [];
         var originalToCanonicalURLMap = {};
         for (var i = 0; i < map.sources.length; ++i) {
             var originalSourceURL = map.sources[i];
-            var href = (map.sourceRoot ? map.sourceRoot + "/" : "") + originalSourceURL;
+            var sourceRoot = map.sourceRoot || "";
+            if (sourceRoot && !sourceRoot.endsWith("/"))
+                sourceRoot += "/";
+            var href = sourceRoot + originalSourceURL;
             var url = WebInspector.ParsedURL.completeURL(this._sourceMappingURL, href) || href;
             originalToCanonicalURLMap[originalSourceURL] = url;
             sources.push(url);
             this._sources[url] = true;
 
             if (map.sourcesContent && map.sourcesContent[i])
                 this._sourceContentByURL[url] = map.sourcesContent[i];
         }
 
         var stringCharIterator = new WebInspector.SourceMap.StringCharIterator(map.mappings);
@@ skipping 108 matching lines @@
     },
 
     /**
      * @return {boolean}
      */
     hasNext: function()
     {
         return this._position < this._string.length;
     }
 }