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In this Cl, we reuse the "backend" of the imperative Shadow DOM Distribution API
for a new custom element, <my-detail>/<my-summary>.

In addition, we add reftest and perftests to compare the custom element,
<my-detail>/<my-summary> to built-in custom element, <detail>/<summary>.

perftest result: built-in element (ave)0.635 ms/new element (ave)1.030 ms

Bug: 869308
Change-Id: If3304c65382bc5ad57f4cfe59b1d1a623a22c471
Reviewed-on: https://chromium-review.googlesource.com/1192783
Commit-Queue: Kyoko Muto <kymuto@google.com>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589409}