Yes, should be OK to remove that now.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fdc7de4c4f751e04414f49cb24957fc12fece162

commit fdc7de4c4f751e04414f49cb24957fc12fece162
Author: Kyoko Muto <kymuto@google.com>
Date: Thu Aug 09 09:02:57 2018

Remove SlotsInFlatTree Runtime flag

BUG= 859429 

Change-Id: Iaa998db509f9c9b4a38c2af1fa3c6be576e2d27f
Reviewed-on: https://chromium-review.googlesource.com/1160068
Commit-Queue: Kyoko Muto <kymuto@google.com>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581833}
[modify] https://crrev.com/fdc7de4c4f751e04414f49cb24957fc12fece162/third_party/blink/renderer/core/css/css_computed_style_declaration_test.cc
[modify] https://crrev.com/fdc7de4c4f751e04414f49cb24957fc12fece162/third_party/blink/renderer/core/dom/flat_tree_traversal_test.cc
[modify] https://crrev.com/fdc7de4c4f751e04414f49cb24957fc12fece162/third_party/blink/renderer/core/dom/node.cc
[modify] https://crrev.com/fdc7de4c4f751e04414f49cb24957fc12fece162/third_party/blink/renderer/core/html/html_slot_element.cc
[modify] https://crrev.com/fdc7de4c4f751e04414f49cb24957fc12fece162/third_party/blink/renderer/core/html/html_slot_element.h
[modify] https://crrev.com/fdc7de4c4f751e04414f49cb24957fc12fece162/third_party/blink/renderer/platform/runtime_enabled_features.json5