DevTools: automatic mapping detection is broken for cases with specific file paths.

third_party/WebKit/Source/devtools/front_end/workspace/FileSystemMapping.js

Index: third_party/WebKit/Source/devtools/front_end/workspace/FileSystemMapping.js
diff --git a/third_party/WebKit/Source/devtools/front_end/workspace/FileSystemMapping.js b/third_party/WebKit/Source/devtools/front_end/workspace/FileSystemMapping.js
index f1a185167cde32a2eb2a6e0c34340aee91551a66..337d2136d33e7732ff3aec48f0896733020a40f8 100644
--- a/third_party/WebKit/Source/devtools/front_end/workspace/FileSystemMapping.js
+++ b/third_party/WebKit/Source/devtools/front_end/workspace/FileSystemMapping.js
@@ -311,9 +311,14 @@ WebInspector.FileSystemMapping.prototype = {
             if (filePathCharacter === "/")
                 commonPathSuffixLength = i;
         }
-        var pathPrefix = filePath.substring(fileSystemPath.length, filePath.length - commonPathSuffixLength);
+        var from = fileSystemPath.length;
+        var to = filePath.length - commonPathSuffixLength;
+        var pathPrefix = filePath.substring(from, to);
         var urlPrefix = url.substr(0, url.length - commonPathSuffixLength);
-        this.addFileMapping(fileSystemPath, urlPrefix, pathPrefix);
+        if (to >= from)
+            this.addFileMapping(fileSystemPath, urlPrefix, pathPrefix);
+        else
+            this.addFileMapping(fileSystemPath, urlPrefix + pathPrefix, "/");
     },
 
     resetForTesting: function()