[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.

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