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This is a preparation of launching incremental shadow dom.

After I confirm that this doesn't cause any regression in bots,
I'll ship this, announcing it to blink-dev briefly, in a few days.

Bug: 776656
Change-Id: I2632a42c2cb9acaf6462600ac236dc9b8bff32ed
Reviewed-on: https://chromium-review.googlesource.com/1056896
Commit-Queue: Hayato Ito <hayato@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558607}