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A previous CL added an opt-in for declarative Shadow DOM, and
this test needs to activate that opt in.

Fixed: 1146530
Change-Id: Iefa585f0a4c83208c6aa977fc65a85ebf6fb4a8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523734
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825239}