[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/GetDirectoryListingTask.cpp

@@ -340,9 +340,14 @@ GetDirectoryListingTaskParent::IOWork()
     nsCOMPtr<nsIFile> currFile = do_QueryInterface(supp);
     MOZ_ASSERT(currFile);
 
-    bool isSpecial, isFile;
-    if (NS_WARN_IF(NS_FAILED(currFile->IsSpecial(&isSpecial))) ||
-        isSpecial) {
+    bool isLink, isSpecial, isFile;
+    if (NS_WARN_IF(NS_FAILED(currFile->IsSymlink(&isLink)) ||
+                   NS_FAILED(currFile->IsSpecial(&isSpecial))) ||
+        // Although we allow explicit individual selection of symlinks via the
+        // file picker, we do not process symlinks in directory traversal.  Our
+        // specific policy decision is documented at
+        // https://bugzilla.mozilla.org/show_bug.cgi?id=1813299#c20
+        isLink || isSpecial) {
       continue;
     }
     if (NS_WARN_IF(NS_FAILED(currFile->IsFile(&isFile)) ||