[DOM] Filter out symlinks for webkitdirectory.

This is effectively a back-out of the following patches from Bug 1274959
except we add comments and test coverage:

- Part 1 which made the DirectoryListingTask include symlinks in the
  results as exposed by Directory.getFilesAndDirectories.
- Part 3 which made GetFilesHelper include symlinks in the results.

Test coverage for getFilesAndDirectories is provided by
dom/filesystem/tests/test_basic.html by way of changes to its included
file dom/filesystem/tests/filesystem_commons.js and changes to the
createTreeFile helper in dom/filesystem/tests/script_fileList.js.
Test coverage for GetFilesHelper is provided by
dom/filesystem/tests/test_webkitdirectory.html and changes to the
createTestFile helper in dom/filesystem/tests/script_fileList.js.

Commenting out either of the `isLink` test in the relevant C++ code will
cause the given tests to fail on non-windows platforms.

dom/filesystem/tests/filesystem_commons.js

@@ -38,6 +38,10 @@ function test_getFilesAndDirectories(aDirectory, aRecursive, aNext) {
           if (data[i] instanceof File) {
             is(data[i].webkitRelativePath, createRelativePath(dir, data[i]), "File.webkitRelativePath should be called: parentdir.path + '/' + file.name: " + data[i].webkitRelativePath);
           }
+          ok(
+            !data[i].webkitRelativePath.endsWith("symlink.txt"),
+            "We should never see a path ending with symlink.txt, our symlink sentinel."
+          );
         }
       }
     );