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As the first step of implementing "Imperative Shadow DOM Distribution API",
this CL makes element.attachShadow take an optional parameter, |slotting|,
which should be 'manual' or 'auto'.

This CL also introduces new Runtime flag, ManualSlotting, to guard the new
feature.

BUG=869308

Change-Id: I71b115886c07c0090bef16b66da48c88e723a622
Reviewed-on: https://chromium-review.googlesource.com/1166767
Commit-Queue: Kyoko Muto <kymuto@google.com>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582524}