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Before shipping Shadow DOM v1, we have to remove this feature because Node.rootNode
is guarded by ShadowDOMV1 flag.

See also https://bugs.chromium.org/p/chromium/issues/detail?id=608006.
Ship of Node.rootNode was reverted there.

BUG=531990

Review-Url: https://codereview.chromium.org/2106683002
Cr-Commit-Position: refs/heads/master@{#402697}