Convert V0 UA shadow in <marquee> to use V1

<marquee> was implemented in V0 UA shadowroot with <content>
element, obviously for wrapping the moving parts inside its
element, and used style/animation to move.

This CL replaces <content> with <slot> to use V1 UA shadowroot.

Bug: 787717, 788625
Change-Id: Ic2bbd8e1536a4736fe0f1e761eecbf7841e8a596
Reviewed-on: https://chromium-review.googlesource.com/798871
Commit-Queue: Takayoshi Kochi <kochi@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#529178}
2 files changed