Revert of Ship Node.rootNode (patchset #2 id:20001 of https://codereview.chromium.org/1783693002/ )

Reason for revert:
Turns out that the change is not Web-compatible. See https://bugs.chromium.org/p/chromium/issues/detail?id=608006 for details.

Original issue's description:
> Ship Node.rootNode
>
> This CL ships Node.rootNode independently from other ShadowDOM features, in response to requests from web developers.
> LGTM-ed on this thread :  https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/LpXX7Svx8IY
>
> BUG=531990
>
> Committed: https://crrev.com/9ca99ec409a1d6f7baf6dd3cd6929e81c967cac1
> Cr-Commit-Position: refs/heads/master@{#380363}

TBR=tkent@chromium.org,hayato@chromium.org,kochi@chromium.org,yuzus@chromium.org
BUG=531990, 608006

Review-Url: https://codereview.chromium.org/1945823004
Cr-Commit-Position: refs/heads/master@{#391723}
(cherry picked from commit 08405679e3c56b833b4c6da3135b30485fad279b)

Review URL: https://codereview.chromium.org/1962403004 .

Cr-Commit-Position: refs/branch-heads/2704@{#492}
Cr-Branched-From: 6e53600def8f60d8c632fadc70d7c1939ccea347-refs/heads/master@{#386251}
3 files changed